feat: Mypy config

[nstarman]

Fixes #147

Plz don't squash.

[nstarman]

Hmm. Not sure what changed with python check_linter_assertions.py tests/typechecked.

[wesselb]

@nstarman The error says that the script expected mypy to throw an error saying

no overload variant of "f" matches argument type "float"

but mypy didn’t. If you run mypy on test_overload.py, do you see that error? Maybe the additional type hints changed mypys error.

Plz don't squash.

Ah, you not like to squash and merge? Would you prefer another way?

[nstarman]

I normally like Squash & Merge, but for this PR I was trying to keep each commit a distinct contribution: the configuration, then adding typing to each module. You can Squash & Merge, of course, I was just suggesting that for this PR a normal merge was appropriate.

[wesselb]

@nstarman Ah, that makes total sense! Thanks for the explanation. I’m also happy to merge without squashing if the commits are nicely structured. I don’t feel strongly any particular way. :)