Add typehints and fix nullable types

Gonna need a CR for the GroupMembership changes

All right, I feel like this is more than enough for a next major. I'd like to have typehints and return hints everywhere, but that will make he new major another massive hurdle again so let's gradually convert our code instead.

Added a CR for the membership loading, we should create a follow-up to deprecate this in 3.3.x and 2.3.x

Automatically closed - issue fixed for 2 weeks with no activity.