test: add type hint test

[mschoettle]

Follow-up to #754 and #757 to catch type hint errors in the future.

[mschoettle]

@last-partizan The failing test is because a lot of type hints use the union operator | which is not supported on Python 3.9.

[last-partizan]

@mschoettle that's interesting. But those files are using from future import __annotations__, so it's working for type-checkers but not for runtime?

If you don't care about python 3.9, then add @pytest.mark.skip based on python version.

[mschoettle]

@mschoettle that's interesting. But those files are using from future import annotations, so it's working for type-checkers but not for runtime?

If you don't care about python 3.9, then add @pytest.mark.skip based on python version.

Yes, it is odd. Good idea, I added @pytest.mark.skipif.