This repo is now testing only with Python 3, so let's make
a few cleanups:
- Remove py2 stanza from setup.py
- Remove obsolete sections from setup.cfg
- Update classifiers
- Switch to using sphinx-build
- Remove install_command from tox.ini, the default is fine
- Remove Babel from requirements, it's not needed for running.
- Remove obsolete babel.cfg
- Use TOX_CONSTRAINTS_FILE instead of obsolete UPPER_CONSTRAINTS_FILE.
- Update hacking to current 3.1.0 version
- Remove use of six library
Change-Id: I233dfc7a06cbd2e098499629f63d0b8f2db5258e
4195ae11b2