Further py27 cleanup support

This repo is now testing only with Python 3, so let's make
a few cleanups:
- Remove obsolete sections from setup.cfg
- Update requirements, no need for python_version anymore
- Switch to using sphinx-build
- Use newer openstackdocstheme and Sphinx versions
- Switch to hacking 3.0, fix problems found

Change-Id: I499bb071847dd006d4045faf2325f588d07b4ffe
5 files changed