test: create assert_not_equal util and add to where imports are needed

In the functional tests there are lots of cases where we assert != which
this new util will replace, we also are adding the imports and the new assertion

test/functional/rpc_psbt.py

@@ -37,6 +37,7 @@ from test_framework.script import CScript, OP_TRUE
 from test_framework.script_util import MIN_STANDARD_TX_NONWITNESS_SIZE
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import (
+    assert_not_equal,
     assert_approx,
     assert_equal,
     assert_greater_than,
@@ -292,7 +293,7 @@ class PSBTTest(BitcoinTestFramework):
         processed_finalized_psbt = self.nodes[0].walletprocesspsbt(psbt=psbtx, finalize=True)
         finalized_psbt = processed_finalized_psbt['psbt']
         finalized_psbt_hex = processed_finalized_psbt['hex']
-        assert signed_psbt != finalized_psbt
+        assert_not_equal(signed_psbt, finalized_psbt)
         assert finalized_psbt_hex == finalized_hex
 
         # Manually selected inputs can be locked: