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: Ie555138c596a5b343b8a1bcf7be44cd493bd2916

openstack/designate-dashboard,openstack/designate-dashboard,openstack/designate-dashboard