Add type annotations & type checking

[Mr0grog]

This is not high priority, but I think it might be useful to add type annotations here. As library, users might appreciate having typing information available (especially now that we have our own return types for everything now, like Memento, rather than returning values directly from requests).

As we add annotations, we should also add type checking to CI, probably with Mypy or Pyright.

[lion-sz]

I'm actually interested in this. I've recently looked into mypy for another project of mine but decided against it.
So I'd be interested in implementing / working on the type annotations.

Did you plan on adding the annotations as you go or going through the entire project?

[Mr0grog]

Probably as we go, the rate of code change here isn’t high, but I don’t want to block anything thing on one giant change.

[Mr0grog]

Quick note here: I've started adding annotations in all new code and am going to try and enforce that on all PRs going forward. I have not added type checking yet (PRs welcome!).

I don't necessarily expect to add types to everything in time for v0.5.0 (shipping this month), but would like to get everything done sometime before spring 2024.