This repo is now testing only with Python 3, so let's make
a few cleanups:
- Remove obsolete sections from setup.cfg
- Remove babel.cfg, this is not needed here at all
- Update requirements, no need for python_version anymore
- Switch to using sphinx-build
- Use newer openstackdocstheme and Sphinx versions
- Remove install_command from tox.ini, the default is fine
- Use TOX_CONSTRAINTS_FILE, UPPER_CONSTRAINTS_FILE is obsolete
Change-Id: I57d447e034c088c83fdd5fb8854dbd9826a200a8
zhurong