Added a missing test method in tests/migrations/test_writer.py.

            SettingsReference("someapp.model", "AUTH_USER_MODEL"),

            ("settings.AUTH_USER_MODEL", {"from django.conf import settings"})

        )

    def test_serialize_iterators(self):

        self.assertSerializedResultEqual(

            ((x, x * x) for x in range(3)),

            ("((0, 0), (1, 1), (2, 4))", set())