tox: Use python3 within tempestenv
tox: Use python3 within tempestenv

If3ff71dbe636ef98072b15f6599e6e641c222f30 had switched this to python3.6
but this breaks on distros where 3.6 is no longer provided by default,
such as Fedora.

Closes-Bug: #1861308
Change-Id: I0e0b9e88eea2499cb1b9727a7787c050c869c819

cisco-openstack/tempest,openstack/tempest,cisco-openstack/tempest,openstack/tempest