Sync sphinx version pinning from requirements repo

Change-Id: I02b808ec6224cab437654b15b8ad46a255ef93f2
Remove tox locale overrides

* tox.ini: The LANG, LANGUAGE and LC_ALL environment overrides were
introduced originally during the testr migration in an attempt to be
conservative about the possibility that locale settings in the
calling environment could cause consistency problems for test runs.
In actuality, this should be unnecessary and any place where it does
cause issues ought to be considered an actual bug. Also, having
these in the configuration actively causes older pip to have
problems with non-ASCII content in some package metadata files under
Python 3, so drop it now.

Change-Id: Ia910ded1789a1a8fac7d82f1ce97300ad7bfefda
Closes-Bug: #1277495
Merge "Sync sphinx version pinning from requirements repo"
Merge "Remove tox locale overrides"