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/sql/plpython_trigger.sql

@@ -69,10 +69,10 @@ CREATE TABLE trigger_test
 
 CREATE FUNCTION trigger_data() returns trigger language plpythonu as $$
 
-if TD.has_key('relid'):
+if 'relid' in TD:
 	TD['relid'] = "bogus:12345"
 
-skeys = TD.keys()
+skeys = list(TD.keys())
 skeys.sort()
 for key in skeys:
 	val = TD[key]