devel/py-mypy: add new port

Mypy is an optional static type checker for Python. You can add type hints to
your Python programs using the standard for type annotations introduced in
Python 3.5 (PEP 484), and use mypy to type check them statically. Find bugs in
your programs without even running them!

PR: 209871
Submitted by: Florian Limberger (flo@snakeoilproductions.net)