PEP-561 & PEP-484 style type support

It would be really great to add type annotations to this module so that downstream consumers don't need to either disable it from their type checkers, or do their own limited-scope stubbing.

There's some indication that there's demand (other than mine) from this stackoverflow: https://stackoverflow.com/questions/57785471/why-does-mypy-think-library-imports-are-missing

I'm more than up for helping out with this is this something the maintainers are interested in, otherwise typeshed is a potential alternative (though keeping it inline would be better for maintainability by far!)

I searched the issues as best I could and I don't believe this is duplicate, so at the very least this can act as a good gauge for how many people are interested in this as a feature!