Drop python 2 and add annotations

[Kludex]

Are there thoughts on what the title proposes? I can open a PR for it, if wanted.

[rabernat]

Thanks for this great package.

The fact that it does not distribute typing stubs causes some problems for downstream packages that want to use static typing. The typical use case is around ObjectID -- see e.g. python/mypy#6701 (comment).

I strongly support the suggestions to add type annotations and also to distribute stub files as part of the package