test: use zero-argument super() shortcut (Python 3.0+)

as defined in PEP 3135:

"The new syntax:

    super()

is equivalent to:

    super(__class__, <firstarg>)

where __class__ is the class that the method was defined in, and <firstarg> is
the first parameter of the method (normally self for instance methods, and cls
for class methods)."

test/functional/wallet_txn_clone.py

@@ -29,7 +29,7 @@ class TxnMallTest(BitcoinTestFramework):
 
     def setup_network(self):
         # Start with split network:
-        super(TxnMallTest, self).setup_network()
+        super().setup_network()
         disconnect_nodes(self.nodes[1], 2)
         disconnect_nodes(self.nodes[2], 1)