Add test cases for pow that are meant to fail a type check

[AlexWaygood]

No description provided.

[AlexWaygood]

Well, I officially have no idea what I'm doing with these pyrightconfig files 🙃 any help in this regard would be very welcome

[srittau]

Could we move the changes to mypy_test to a separate PR? I think they are individually useful.

[AlexWaygood]

Great, looks like both mypy and pyright will fail if there's an unused type: ignore in the test_cases directory, with the latest version of this patch applied.

• Here's what the mypy failure looks like: https://github.com/python/typeshed/runs/6271321242?check_suite_focus=true
• Here's what the pyright failure looks like: https://github.com/python/typeshed/runs/6271329437?check_suite_focus=true