Add setup.cfg to allow py2dsc to debianize package

By adding this file to the package, it allows a debian package for websocket-client
to be created this easily:

   python setup.py sdist
   cd dist
   py2dsc websocket-client-0.13.0.tar.gz
   cd deb_dist
   cd websocket-client-0.13.0
   debuild -uc -us

Without this file, py2dsc fails.