Update docs job to avoid setup.py entrypoint

The docs builds were previously configured to use PBR's sphinx
entrypoint. However, in newer versions of PBR that no longer exists and
the docs builds are failing. This commit updates the tox job to just
directly call sphinx-build to fix the docs builds locally and in CI.

tox.ini

@@ -55,7 +55,7 @@ extras =
   sql
 commands =
   doc8 -e .rst doc/source CONTRIBUTING.rst README.rst
-  python setup.py build_sphinx
+  sphinx-build -W -T --keep-going -b html doc/source doc/build/html
 
 [testenv:releasenotes]
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html