Update docs building, cleanup a bit:
* Switch to sphinx-build
* Update requirements for Sphinx and openstackdocstheme for
python 3, create doc/requirements.txt
* Remove unneeded doc and translation sections from setup.cfg
* Remove install_command, it's unneeded, the default is fine.
* Move constraints into deps, use TOX_CONSTRAINTS instead of
* Update to hacking 3.0, fix problems found