adding types annotations and py 2.7 support

[aogier]

dear maintainers, since I have recently started using this library a lot I would like to contribute a patch with types annotations so I stop putting comments for mypy :D

Seeing the code I find some parts with workarounds to support python 2.7 and although in setup.py I see a requirement >= 3.7 I am not sure if this compatibility is somehow a requirement of the project, which in this case would stop me from contributing on that.

What do you think? Are you interested in such a contribution ? I could also write a stub and put on the usual typeshed repo but I'd prefer to work on this upstream.

Thank you, regards