Update README section on Python3 support
Update README section on Python3 support

Now that we have a gating Python 3 job running on all tempest commits we
can update the language in the Readme about the Python 3 section. This
commit does just that.

Change-Id: I2a1d35cae6c28a0b427ffd142b8a3d2f25a6dbe7

vedujoshi/tempest,masayukig/tempest,openstack/tempest,Juniper/tempest,masayukig/tempest,cisco-openstack/tempest,cisco-openstack/tempest,openstack/tempest,Juniper/tempest,vedujoshi/tempest