Use py3 as the default runtime for tox
Use py3 as the default runtime for tox

Moving on py3 as the default runtime for tox to avoid to update
this at each new cycle.

Change-Id: I1ed9e545750e8db264cc4381a69a3f61ba8ddf0f

openstack/vitrage-dashboard,openstack/vitrage-dashboard,openstack/vitrage-dashboard,openstack/vitrage-dashboard