Replace obsolete build-sphinx job with tox-docs which runs "tox -e docs" directly. Update sphinx requirements for python 3. Depends-On: https://review.opendev.org/660026 Change-Id: Ibb22861e7b1e0e1e40f155043db8b3e554eaf246
Change-Id: Ia5dd012d8f6debd0e7cbd7b19747d6b5b43521a6