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: Ic4f1253a369be2b8f8d2a66b8d3f67ab35e5daf7

openstack/stackviz,openstack/stackviz,openstack/stackviz