Use general py3 tox env for default tox run
Use general py3 tox env for default tox run

different distros already come with different minor versions of Python3,
so let's use general 'py3' tox env when invoking tox without specific
env.

This is just a convenience for developers and will not affect gates,
as they use specific tox envs, all of them pre-generated by tox by
default.

Change-Id: Ife1d1b66e5ebf698f16a5b830604117a6e111107

openstack/ironic-inspector,openstack/ironic-inspector