Move from pygithub3 to github3.py.

The impetuts for this change was built-in OAuth support; pygithub3 has a pull
request for it[0] that's been sitting open for 5 months.

github3.py also has a nicer API and the maintainer is much more active, so it's
a win all around.

[0]: https://github.com/copitux/python-github3/pull/30
Use OAuth instead of hardcoded username/password.