Simplify our setup.py

Currently our setup.py is quite intricate and contains more logic than I am comfortable with.

An initial round of changes in #842 makes sure we no longer depend on the deprecated distutils package, but I'd like to go further and make our setup.py as close as possible as a vanilla Cython-based project.

AFAICT one reason we cannot simply use cythonize is because of our generated av/config.h header. I'd argue this header is not useful because:

• The git commit string cannot always be retrieved. If you are building from a tarball retrieved from PyPI for instance, this is not available.
• The PyAV version does not need to be read from a VERSION file: we could simply have an av/version.py which would be our single source of truth for the current version. This can easily be read from setup.py

Thoughts @mikeboers ?