Replace build-sphinx job with tox-docs (using template
build-openstack-docs-pti) job as best practice for building docs.
Update tox.ini for this and remove now obsolete section from setup.cfg.
Fix doc building: Remove option from doc/source/conf.py for an unused
directory.
The tox.ini file already contains python3 for releasenotes, update the
job now to do this as well.
Change-Id: I7552c802a99f0338ff6a5a3bba1f24902c55bed4
e370fc937c