PL/Python: Fix tests for older Python versions

Commit 8561e4840c neglected to handle
older Python versions that don't support the "with" statement.  So write
the tests in a way that older versions can handle as well.

src/pl/plpython/sql/plpython_transaction.sql

@@ -79,8 +79,9 @@ SELECT transaction_test4();
 
 -- commit inside subtransaction (prohibited)
 DO LANGUAGE plpythonu $$
-with plpy.subtransaction():
-    plpy.commit()
+s = plpy.subtransaction()
+s.enter()
+plpy.commit()
 $$;