Type Hinting

[tjdevries]

We could use mypy (http://mypy-lang.org/) and add type hinting.

I've been wanting to try and do it for a whole project for a bit. We could add it to the testing suite.

[sprout42]

I intended to do that originally but had a hard time figuring out how. Now that mypy is stable it's probably a good time.

I'm a big fan of strong typing, I find it helps avoid most programming errors. Since this module is all about typing and data transformation, it'd be a cool chance to try it out.