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
- Cleanup */source/conf.py to remove now obsolete content.
- Remove install_command from tox.ini, the default is fine
- Fix problems found by sphinx-build
Change-Id: I258ba61fb3c2c5e83e71109dab941e304dde3f76
44988971ef