Follow the PTI for docs

Use sphinx-build instead of the pbr sphinx extention for building docs
as instructed by the PTI[1].

It fixes the header formatting for the index page, as the headers weren't
rendering at all.

[1] https://governance.openstack.org/tc/reference/pti/python.html

Change-Id: Ibac2b45ecfab4a7e575d097ecb9fc2c5e57b81cf

doc/source/index.rst

@@ -6,7 +6,8 @@
 Welcome to openstack's documentation!
 ========================================================
 
-Contents:
+Contents
+--------
 
 .. toctree::
    :maxdepth: 2
@@ -17,7 +18,7 @@ Contents:
    contributing
 
 Indices and tables
-==================
+-------------------
 
 * :ref:`genindex`
 * :ref:`modindex`

tox.ini

@@ -26,7 +26,7 @@ commands = python setup.py test --coverage --testr-args='{posargs}'
 
 [testenv:docs]
 basepython = python3
-commands = python setup.py build_sphinx
+commands = sphinx-build -W -b html -d doc/buld/doctrees doc/source doc/build/html
 
 [testenv:releasenotes]
 basepython = python3