Update docs building

Update docs building:
* Switch to sphinx-build
* Update requirements for Sphinx and openstackdocstheme for
  python 3, create doc/requirements.txt file.
* Remove unneeded doc and translation sections from setup.cfg
* Remove install_command, it's unneeded, the default is fine.
* Use TOX_CONSTRAINTS instead of obsolete UPPER_CONSTRAINTS

Change-Id: I6ff4427014fc967a6f7b28fc6cbe5a0f4eca601c
4 files changed