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.

Change-Id: I7d0fc0d56f03a5c77cb02800e6a4abaa5ef59bc5

openstack/stackviz,openstack/stackviz,openstack/stackviz