Fix regression test to also work with Python 2.

Per buildfarm.

src/pl/plpython/expected/plpython_composite.out

@@ -572,7 +572,7 @@ SELECT * FROM return_record_2('v3') AS (v1 int, v2 int, v3 int);
 -- multi-dimensional array of composite types.
 CREATE FUNCTION composite_type_as_list()  RETURNS type_record[] AS $$
   return [[('first', 1), ('second', 1)], [('first', 2), ('second', 2)], [('first', 3), ('second', 3)]];
-$$ LANGUAGE plpython3u;
+$$ LANGUAGE plpythonu;
 SELECT * FROM composite_type_as_list();
                                composite_type_as_list                               
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