split docs requirements out into doc/requirements.txt and change the
helper tox docs env to be what the gate is going to be running.
As part of doing this, we can re-enable the enforcer code. However, it
doesn't seem to be working, so leave it as info rather than warn for
now.
Set the jobs and the tox env to use python3. The enforcer module wants
to use ifilterfalse/filterfalse which have different names in 2 vs. 3.
There's no reason to NOT run sphinx in python3.
Change-Id: I358db11b130b909084f7a9e8925477e931d87117
da99fae94d