Update docs building

Update docs building:
* Switch to sphinx-build
* Update requirements for Sphinx and openstackdocstheme for
  python 3
* Remove unneeded doc and translation sections from setup.cfg

Change-Id: Ic1a93606cb73778fc43e58b85ad65bee46c3c631
diff --git a/tox.ini b/tox.ini
index 59764db..640880d 100644
--- a/tox.ini
+++ b/tox.ini
@@ -24,7 +24,7 @@
 commands = {posargs}
 
 [testenv:docs]
-commands = python setup.py build_sphinx
+commands = sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:releasenotes]
 commands =