This repo is now testing only with Python 3, so let's make
a few cleanups:
- Remove obsolete sections from setup.cfg
- Use newer openstackdocstheme,reno and Sphinx versions
- Remove install_command from tox.ini, the default is fine
- Use TOX_CONSTRAINTS_FILE, UPPER_CONSTRAINTS_FILE is obsolete
- Update hacking to 3.0.1, fix warnings found
- Remove babel.cfg, it's not needed anymore
Change-Id: I60edb7e44e4a8222f564f30157eb31288b85b422
Andreas Jaeger