tests: remove byte.hex() to keep compatibility

Use test_framework.util.bytes_to_hex_str() instead of bytes.hex() that
new in Python 3.5, to support minimum version of Python(test).

test/functional/wallet_txn_clone.py

@@ -8,6 +8,7 @@ import io
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import (
     assert_equal,
+    bytes_to_hex_str as b2x,
     connect_nodes,
     disconnect_nodes,
     sync_blocks,
@@ -81,7 +82,7 @@ class TxnMallTest(BitcoinTestFramework):
 
         # Use a different signature hash type to sign.  This creates an equivalent but malleated clone.
         # Don't send the clone anywhere yet
-        tx1_clone = self.nodes[0].signrawtransactionwithwallet(clone_tx.serialize().hex(), None, "ALL|ANYONECANPAY")
+        tx1_clone = self.nodes[0].signrawtransactionwithwallet(b2x(clone_tx.serialize()), None, "ALL|ANYONECANPAY")
         assert_equal(tx1_clone["complete"], True)
 
         # Have node0 mine a block, if requested: