Add py36 and py37 tox envs
Add py36 and py37 tox envs

With python3.6 and python3.7 jobs added to the Zuul jobs, it makes sense
to add this to the tox config too so that developers can run those
tests as well.

Change-Id: Iccc84a4336302b49a2d5e9a82518c06914f4794a

openstack/oslo.policy