Consider type checking

[cblegare]

While investigating #649, I thought to myself that this kind of issue is quite hard to prevent without type checking, and upgrading libs can get quite time consuming or risky. Since click and many libraries are getting more involved in type checking, it becomes simpler et more valuable to check type consistency.

Since this might also involve type checking libtmux, that might be a lot of work

[tony]

@cblegare We will! and welcome PRs.

We are removing python 2.7 so will have access to python 3.x's type annotations