Add typing annotations

[Kroppeb]

Hi

I'm looking into helping out with solving some issues in pipenv, and was diving through some of the code.

I noticed that there a lot of code doesn't have type annotations or docs with type information. Is this an intentional choice, or more a lack of necessity and time?

Having typing information would make navigating the code quite a bit more pleasant :)

Ps. Is there a place where the pipenv team discusses development things outside of github issues?