This repo is now testing only with Python 3, so let's make
a few cleanups:
- Remove python 2.7 stanza from setup.py
- Add requires on python >= 3.6 to setup.cfg so that pypi and pip
know about the requirement
- Remove obsolete sections from setup.cfg
- Update classifiers
- Remove install_command from tox.ini, the default is fine
- Use TOX_CONSTRAINTS_FILE instead of obsolete UPPER_CONSTRAINTS_FILE.
- Update hacking to current version 3.1.0, fix problems found.
Change-Id: I86c18b4c73f9b3dc0b3d804559d825dac5a78436
6d253ca7f1