Add type hints

[jan-janssen]

No description provided.

[liamhuber]

With the disclaimer that I haven't verified that the value of the hints is correct in any way, looks good. I'd give a friendly reminder that it's helpful to sub-hint iterables (dict->dict[str, int] etc.), but since that's already done in some places I'm assuming in the rest there is no super clear hint -- anyhow, even without the sub-hint it's already a mergable improvement.

Only necessary change is that all hints of the variety var: type_hint = None need to be replaced with var: typing.Optional[type_hint] = None (or type_hint | None if you're only supporting newer python versions, but I forget the cutoff). I'm approving despite the requested change because there is zero need for re-review after such a simple change.