Migrate from 2to3 to a unified code base

While #111 (closed) can be fixed easily sticking to an older version of setuptools, it's still a good idea to refresh a bit the code making it compatible with Python 2 and 3 at the same time.

This does not mean fix the fact that the nightly builds do not work under Python 3, but it could be a starting point to decommission Python 2 in the old version of the nightly builds.