* Remove usage of six, this repo is using python3 only.
* Remove settings from conf.py that are default in
newer openstackdocstheme versions.
* Remove obsolete build_sphinx sections from setup.cfg, sphinx-build
does not need this. Remove ignored warnerror.
Change-Id: I7b0625a71d81305d90157e42fc6a5065c9be47d1