Update docs building:
* Switch to sphinx-build
* Update requirements for Sphinx and openstackdocstheme for
python 3, create doc/requiremetns
* Remove unneeded doc and translation sections from setup.cfg
* Remove install_command, it's unneeded, the default is fine.
* Move constraints to deps, use TOX_CONSTRAINTS instead of obsolete
UPPER_CONSTRAINTS
* Remove now unneeded placeholder file
Change-Id: I649e6fa86c17262698cc8a01d042a87b020239d9
Andreas Jaeger