'python setup.py clean' does not clean build files

[stewartadam]

I ran python setup.py build and at the end it informed my I was missing mp3 decode support, so I installed the dependency and noticed a few issues:

1. Re-running python setup.py build shows the updated dependency map but files are not rebuild (i.e. MP3 decode support was not added in)
2. Running python setup.py clean does not actually clean build files; running python setup.py build afterwards still doesn't build anything. Manually removing the __pycache__ and build folders fixed the issue.

[maphew]

Also see pypa/setuptools#1347