Move type hints from stubs to be inline

[sarahmonod]

As was raised with #58, having the type hints in a separate file, which was helpful when we supported python2.7 so that we could use a more reasonable syntax, is no longer helpful and makes it hard to evolve
the library without introducing type issues.

Moving the type hints to be inline would make it easier to change them if and when needed.

[godlygeek]

Done in #65