This repo is now testing only with Python 3, so let's make
a few cleanups:
- Remove python 2.7 stanza from setup.py
- Remove obsolete sections from setup.cfg
- Update classifiers
- Update requirements, no need for python_version anymore
- Cleanup doc/source/conf.py to remove now obsolete content,
use sphinxcontrib.apidoc to build module index again (this
seems to have been broken when switching to sphinx-build).
- Remove install_command from tox.ini, the default is fine
- Use TOX_CONSTRAINTS_FILE instead of obsolete UPPER_CONSTRAINTS_FILE.
- Update tox.ini for python3 only support.