PEP 561 compliant

[meatballs]

Given that this library makes extensive use of type hints, we should choose and use one of the tools to check those.

We can then and add that tool to the github actions workflow and the pre-commit hooks.

e.g. running mypy against the codebase currently lists 140 errors.

Also, once those are fixed, we should the py.typed file to make the library PEP 561 compliant.

Does anyone have preferences on the type checker? I tend to use mypy, but that's more out of habit than anything else.

[meatballs]

If there's support for doing this, I can work on it once I'm done with #3165

[theref]

i have no opinion, mypy seems like a perfectly reasonable choice