Consistently name sign_tx psbt argument

### hwilib/devices/coldcard.py
@@ -155,7 +155,7 @@ def get_master_fingerprint(self) -> bytes:
         return struct.pack('<I', self.device.master_fingerprint)
 
     @coldcard_exception
-    def sign_tx(self, tx: PSBT) -> PSBT:
+    def sign_tx(self, psbt: PSBT) -> PSBT:
         """
         Sign a transaction with the Coldcard.
 
@@ -172,7 +172,7 @@ def sign_tx(self, tx: PSBT) -> PSBT:
         # For multisigs, we may need to do multiple passes if we appear in an input multiple times
         passes = 1
         if not self.is_edge:
-            for psbt_in in tx.inputs:
+            for psbt_in in psbt.inputs:
                 our_keys = 0
                 for key in psbt_in.hd_keypaths.keys():
                     keypath = psbt_in.hd_keypaths[key]
@@ -181,12 +181,12 @@ def sign_tx(self, tx: PSBT) -> PSBT:
                 if our_keys > passes:
                     passes = our_keys
 
-        if tx.version == 2 and not self._supports_psbt_v2():
-            tx.convert_to_v0()
+        if psbt.version == 2 and not self._supports_psbt_v2():
+            psbt.convert_to_v0()
 
         for _ in range(passes):
             # Get psbt in hex and then make binary
-            fd = io.BytesIO(base64.b64decode(tx.serialize()))
+            fd = io.BytesIO(base64.b64decode(psbt.serialize()))
 
             # learn size (portable way)
             sz = fd.seek(0, 2)
@@ -232,10 +232,10 @@ def sign_tx(self, tx: PSBT) -> PSBT:
 
             result = self.device.download_file(result_len, result_sha, file_number=1)
 
-            tx = PSBT()
-            tx.deserialize(base64.b64encode(result).decode())
+            psbt = PSBT()
+            psbt.deserialize(base64.b64encode(result).decode())
 
-        return tx
+        return psbt
 
     @coldcard_exception
     def sign_message(self, message: Union[str, bytes], keypath: str) -> str:

### hwilib/devices/digitalbitbox.py
@@ -391,17 +391,17 @@ def get_pubkey_at_path(self, path: str) -> ExtendedKey:
         return xpub
 
     @digitalbitbox_exception
-    def sign_tx(self, tx: PSBT) -> PSBT:
+    def sign_tx(self, psbt: PSBT) -> PSBT:
 
         # Create a transaction with all scriptsigs blanked out
-        blank_tx = tx.get_unsigned_tx()
+        blank_tx = psbt.get_unsigned_tx()
 
         # Get the master key fingerprint
         master_fp = self.get_master_fingerprint()
 
         # create sighashes
         sighash_tuples = []
-        for txin, psbt_in, i_num in zip(blank_tx.vin, tx.inputs, range(len(blank_tx.vin))):
+        for txin, psbt_in, i_num in zip(blank_tx.vin, psbt.inputs, range(len(blank_tx.vin))):
             sighash = b""
             utxo = None
             if psbt_in.witness_utxo:
@@ -497,7 +497,7 @@ def sign_tx(self, tx: PSBT) -> PSBT:
 
         # Return early if nothing to do
         if len(sighash_tuples) == 0:
-            return tx
+            return psbt
 
         for i in range(0, len(sighash_tuples), 15):
             tups = sighash_tuples[i:i + 15]
@@ -537,9 +537,9 @@ def sign_tx(self, tx: PSBT) -> PSBT:
 
             # add sigs to tx
             for tup, sig in zip(tups, der_sigs):
-                tx.inputs[tup[2]].partial_sigs[tup[3]] = sig
+                psbt.inputs[tup[2]].partial_sigs[tup[3]] = sig
 
-        return tx
+        return psbt
 
     @digitalbitbox_exception
     def sign_message(self, message: Union[str, bytes], keypath: str) -> str:

### hwilib/devices/jade.py
@@ -374,16 +374,16 @@ def _split_at_last_hardened_element(path: Sequence[int]) -> Tuple[Sequence[int],
     # Sign tx PSBT - newer Jade firmware supports native PSBT signing, but old firmwares require
     # mapping to the legacy 'sign_tx' structures.
     @jade_exception
-    def sign_tx(self, tx: PSBT) -> PSBT:
+    def sign_tx(self, psbt: PSBT) -> PSBT:
         """
         Sign a transaction with the Blockstream Jade.
         """
         # Old firmware does not have native PSBT handling - use legacy method
         if self.PSBT_SUPPORTED_FW_VERSION > self.fw_version.finalize_version():
-            return self.legacy_sign_tx(tx)
+            return self.legacy_sign_tx(psbt)
 
         # Firmware 0.1.47 (March 2023) and later support native PSBT signing
-        psbt_b64 = tx.serialize()
+        psbt_b64 = psbt.serialize()
         psbt_bytes = base64.b64decode(psbt_b64.strip())
 
         # NOTE: sign_psbt() does not use AE signatures, so sticks with default (rfc6979)

### hwilib/devices/ledger.py
@@ -188,7 +188,7 @@ def get_pubkey_at_path(self, path: str) -> ExtendedKey:
         return ExtendedKey.deserialize(xpub_str)
 
     @ledger_exception
-    def sign_tx(self, tx: PSBT) -> PSBT:
+    def sign_tx(self, psbt: PSBT) -> PSBT:
         """
         Sign a transaction with a Ledger device. Not all transactions can be signed by a Ledger.
 
@@ -209,20 +209,20 @@ def legacy_sign_tx() -> PSBT:
             if not isinstance(client, LegacyClient):
                 client = LegacyClient(self.transport_client, self.chain)
             wallet = WalletPolicy("", "wpkh(@0/**)", [""])
-            legacy_input_sigs = client.sign_psbt(tx, wallet, None)
+            legacy_input_sigs = client.sign_psbt(psbt, wallet, None)
 
             for idx, partial_sig in legacy_input_sigs:
-                psbt_in = tx.inputs[idx]
+                psbt_in = psbt.inputs[idx]
                 psbt_in.partial_sigs[partial_sig.pubkey] = partial_sig.signature
-            return tx
+            return psbt
 
         if isinstance(self.client, LegacyClient):
             return legacy_sign_tx()
 
         # Make a deepcopy of this psbt. We will need to modify it to get signing to work,
         # which will affect the caller's detection for whether signing occured.
-        psbt2 = copy.deepcopy(tx)
-        if tx.version != 2:
+        psbt2 = copy.deepcopy(psbt)
+        if psbt.version != 2:
             psbt2.convert_to_v2()
 
         # Figure out which wallets are signing
@@ -374,13 +374,13 @@ def process_origin(origin: KeyOriginInfo) -> None:
                     psbt_in.partial_sigs[yielded.pubkey] = yielded.signature
 
         # Extract the sigs from psbt2 and put them into tx
-        for sig_in, psbt_in in zip(psbt2.inputs, tx.inputs):
+        for sig_in, psbt_in in zip(psbt2.inputs, psbt.inputs):
             psbt_in.partial_sigs.update(sig_in.partial_sigs)
             psbt_in.tap_script_sigs.update(sig_in.tap_script_sigs)
             if len(sig_in.tap_key_sig) != 0 and len(psbt_in.tap_key_sig) == 0:
                 psbt_in.tap_key_sig = sig_in.tap_key_sig
 
-        return tx
+        return psbt
 
     @ledger_exception
     def sign_message(self, message: Union[str, bytes], keypath: str) -> str:

### hwilib/devices/trezor.py
@@ -359,7 +359,7 @@ def get_pubkey_at_path(self, path: str) -> ExtendedKey:
         return xpub
 
     @trezor_exception
-    def sign_tx(self, tx: PSBT) -> PSBT:
+    def sign_tx(self, psbt: PSBT) -> PSBT:
         """
         Sign a transaction with the Trezor. There are some limitations to what transactions can be signed.
 
@@ -382,7 +382,7 @@ def sign_tx(self, tx: PSBT) -> PSBT:
             # Prepare inputs
             inputs = []
             to_ignore = [] # Note down which inputs whose signatures we're going to ignore
-            for input_num, psbt_in in builtins.enumerate(tx.inputs):
+            for input_num, psbt_in in builtins.enumerate(psbt.inputs):
                 assert psbt_in.prev_txid is not None
                 assert psbt_in.prev_out is not None
                 assert psbt_in.sequence is not None
@@ -447,7 +447,7 @@ def ignore_input() -> None:
                     to_ignore.append(input_num)
 
                 # Check for multisig
-                is_ms, multisig = parse_multisig(scriptcode, tx.xpub, psbt_in)
+                is_ms, multisig = parse_multisig(scriptcode, psbt.xpub, psbt_in)
                 if is_ms:
                     # Add to txinputtype
                     txinputtype.multisig = multisig
@@ -533,7 +533,7 @@ def ignore_input() -> None:
 
             # prepare outputs
             outputs = []
-            for psbt_out in tx.outputs:
+            for psbt_out in psbt.outputs:
                 out = psbt_out.get_txout()
                 txoutput = messages.TxOutputType(amount=out.nValue)
                 txoutput.script_type = messages.OutputScriptType.PAYTOADDRESS
@@ -582,7 +582,7 @@ def ignore_input() -> None:
                 if psbt_out.witness_script or psbt_out.redeem_script:
                     is_ms, multisig = parse_multisig(
                         psbt_out.witness_script or psbt_out.redeem_script,
-                        tx.xpub, psbt_out)
+                        psbt.xpub, psbt_out)
                     if is_ms:
                         txoutput.multisig = multisig
                         if not wit:
@@ -593,7 +593,7 @@ def ignore_input() -> None:
 
             # Prepare prev txs
             prevtxs = {}
-            for psbt_in in tx.inputs:
+            for psbt_in in psbt.inputs:
                 if psbt_in.non_witness_utxo:
                     prev = psbt_in.non_witness_utxo
 
@@ -622,20 +622,20 @@ def ignore_input() -> None:
                     prevtxs[ser_uint256(psbt_in.non_witness_utxo.sha256)[::-1]] = t
 
             # Sign the transaction
-            assert tx.tx_version is not None
+            assert psbt.tx_version is not None
             signed_tx = btc.sign_tx(
                 client=self.client,
                 coin_name=self.coin_name,
                 inputs=inputs,
                 outputs=outputs,
                 prev_txes=prevtxs,
-                version=tx.tx_version,
-                lock_time=tx.compute_lock_time(),
+                version=psbt.tx_version,
+                lock_time=psbt.compute_lock_time(),
                 serialize=False,
             )
 
             # Each input has one signature
-            for input_num, (psbt_in, sig) in py_enumerate(list(zip(tx.inputs, signed_tx[0]))):
+            for input_num, (psbt_in, sig) in py_enumerate(list(zip(psbt.inputs, signed_tx[0]))):
                 if input_num in to_ignore:
                     continue
                 for pubkey in psbt_in.hd_keypaths.keys():
@@ -650,7 +650,7 @@ def ignore_input() -> None:
 
             p += 1
 
-        return tx
+        return psbt
 
     @trezor_exception
     def sign_message(self, message: Union[str, bytes], keypath: str) -> str: