Use MyPy in Python 3

MyPy performs static analysis of types using Python 3's type annotations.

As the Python 3 port is a fairly large endeavor, it would be helpful to begin using this tool.

For the purpose of this story, include MyPy in the basic CI testing configuration and implement an annotation or two to get the ball rolling. This may be non-trivial as we have not used MyPy before and, from a quick experiment, I believe it will also attempt to look at the relationship to external packages as well which may not have annotations.