Avoid PytestReturnNotNoneWarning in test_inheriting_class

[graingert]

dask/tests/test_typing.py::test_inheriting_class
  /home/graingert/anaconda3/envs/dask-310/lib/python3.10/site-packages/_pytest/python.py:199: PytestReturnNotNoneWarning: Expected None, but dask/tests/test_typing.py::test_inheriting_class returned <dask.tests.test_typing.Inheriting object at 0x7ff3eff7bee0>, which will be an error in a future version of pytest.  Did you mean to use `assert` instead of `return`?
    warnings.warn(

• Closes #xxxx
• Tests added / passed
• Passes pre-commit run --all-files

[jrbourbeau]

Thanks @graingert! I see this is marked as a draft still -- are there other changes you'd like to add?

@douglasdavis do you have bandwidth to review?

[douglasdavis]

This looks fine, thanks for catching it!