Class S3

Lib/test/test_patma.py:73–74

        class S2(C, collections.UserList, collections.abc.Mapping):
            pass
        class S3(list, C, collections.abc.Mapping):
            pass
        class S4(collections.UserList, dict, C):
            pass
        self.assertEqual(self.check_sequence_then_mapping(S1()), "seq")