Mark sphinx extensions thread safe
Mark sphinx extensions thread safe

This adds the return of some metadata to our sphinx extension setup to
indicate they are thread safe. This is needed to allow consuming
projects to do multithreaded docs builds. In some cases, this can save a
noticeable amount of time in job execution.

Change-Id: I43e8f0c992eaec9f5b669eea4ae551c8a3273588
Signed-off-by: Sean McGinnis <961eeb09b6d45516665e15b82e013282c4faf575@gmail.com>

openstack/stevedore