Make a few cleanups:
- Remove python 2.7 stanza from setup.py
- Update requirements: Remove sphinx, it's not needed for testing, add
pygments which is really needed.
- Update doc/requirements: Remove python 2.7 support
- tox.ini: Remove testing of po files, the infra scripts do this since
a long time
- Update conf.py, no need to import openstackdocstheme anymore.
Change-Id: I9d030eb450f2c7ae74c25b7564a01b8785503e5e
176d24de94