Remove duplicate classifiers.
Merge pull request #710 from berkerpeksag/patch-1

Remove duplicate classifiers.
Add universal Wheel support.

Gunicorn works on both Python 2 and Python 3, so we can create a
universal Wheel distribution.

See https://github.com/pypa/sampleproject/blob/master/setup.cfg for
more information.
Merge pull request #711 from berkerpeksag/wheel-universal

Add universal Wheel support.