Run travis with python2 and python3
We should make sure all of tor's test python scripts run in environments where python3 is the default.
(Scripts are also allowed to explicitly require python3. But they can't depend on python2, because it is end of life on 1 Jan 2020.)
We should be able to copy the python bits of our chutney travis config into Tor.
Let's just pick the newest python 3?