Make PL/Python tests more compatible with Python 3

This changes a bunch of incidentially used constructs in the PL/Python
regression tests to equivalent constructs in cases where Python 3 no longer
supports the old syntax.  Support for older Python versions is unchanged.

src/pl/plpython/expected/plpython_test.out

@@ -10,7 +10,7 @@ select stupid();
 -- test multiple arguments
 CREATE FUNCTION argument_test_one(u users, a1 text, a2 text) RETURNS text
 	AS
-'keys = u.keys()
+'keys = list(u.keys())
 keys.sort()
 out = []
 for key in keys: