Support python 3

This:

- stops allowing Python 3 failures in Travis CI.
- adds Python 3 (and Python 2) classifiers to `setup.py`, so prospective users know what versions are supported, as well as tools like [Python 3 Wall of Superpowers](http://python3wos.appspot.com/), [caniusepython3.com](https://caniusepython3.com/project/GitPython), etc.