Add mypy stub files to PyAV

Created by: tooxo

Overview

To simplify developing and linting with PyAV, stub files should be included. explanation of stubs

Desired Behavior

Every object / method should have defined stubs, which are detectable by IDEs such as PyCharm aswell as typical linters like MyPy.

Example API

Not relevant for this problem (?)

Additional context

For futher explanation see: https://www.python.org/dev/peps/pep-0484/

A possible solution could be using a stub generator like stubgen.