Use mypy

[dcrosta]

I'd rather not use comment-style annotations. And since this is a library, it might make sense to use (and ship) stub files so that others can benefit from the type annotation. I'm not an expert here so looking for opinions and/or need to do research to figure out what's best.

[kyzima-spb]

Many large libraries began to add annotations, such as pymongo.

So I would add annotations, maybe myself =)