Add some typing annotations

[vladdu]

No description provided.

[pawamoy]

If you're OK with it, I've started using from __future__ import annotations, which lets you write str | None instead of Optional[str], and list[str] instead of List[str].

[vladdu]

Sounds good, I will update to use that. Some of these fields should not be optional, really, they are just initialized a bit late. I will see if I can do something about that.

[pawamoy]

Yeah I never know how to handle this case (init to None and set it later). If you find a better way that's great!