Fix pytest.mark.asyncio warning

[adamchainz]

Fix this warning in the test suite:

test_auth_async.py::test_async_authenticate_method_in_sync_context
  test_auth_async.py:142: PytestWarning: The test <Function test_async_authenticate_method_in_sync_context> is marked with '@pytest.mark.asyncio' but it is not an async function. Please remove asyncio marker. If the test is not marked explicitly, check for global markers applied via 'pytestmark'.
    @pytest.mark.skipif(django.VERSION < (3, 1), reason="requires django 3.1 or higher")