Make a few cleanups:
- Remove obsolete sections from setup.cfg
- Remove install_command from tox.ini
- Update to current hacking version; fix warning found
- Remove unused requirements, update to newer versions for python3
Change-Id: I27fa5ef5cb23650e5210f714ce0c3dccdc564e8b
Since pbr already landed and the old version of hacking seems not
work very well with pbr>=2, we should update it to match global
requirement.
Partial-Bug: #1668848
Change-Id: I68ad067b2af0a07d2fb7859986063887ffc13c27