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
- Remove install_command from tox.ini, the default is fine
- Remove babel.cfg, it's not needed anymore.
- Use TOX_CONSTRAINTS_FILE, UPPER_CONSTRAINTS_FILE is obsolete
Change-Id: I235cf10f6a619fa6c41e688a17337385a1034aa2