This repo is now testing only with Python 3, so let's make
a few cleanups:
- Remove python 2.7 stanza from setup.py
- Remove obsolete sections from setup.cfg
- Update requirements, no need for python_version anymore
- Switch to using sphinx-build
- Cleanup doc/source/conf.py to remove now obsolete content.
- Use newer openstackdocstheme version
- Remove install_command from tox.ini, the default is fine
- Update hacking to version 3
Change-Id: I7e9e3993aeb05fb623be531160e418572fa9de54