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
- Use newer openstackdocstheme and Sphinx versions
- Remove install_command from tox.ini, the default is fine
We want to default to running all tox environments under python 3, so
set the basepython value in each environment.
We do not want to specify a minor version number, because we do not
want to have to update the file every time we upgrade python.
We do not want to set the override once in testenv, because that
breaks the more specific versions used in default environments like
py35 and py36.
Signed-off-by: Doug Hellmann <email@example.com>