Our doc requirements have been updated to run documentation builds
using sphinx-build instead of calling tox. The PTI reference for
Python code still contained old recommendations to use
[build_sphinx] in setup.cfg. This updates this Python recommendation
to match what is stated in the full PTI with additions for making
local runs easier through using tox.
Change-Id: Ibdaeefdac6b6bbeef544778d2fa5c3972d180cb8
9ecbfde532