To run tests you need both py2 and py3

[AutomatedTester]

TO run the wpt tests, which only work on py2, the script that runs them is written for py3. This creates an interesting dynamic to make it work.

We need to pick one or other

[jgraham]

Updating wpt/wptrunner to use Python 3 is likely a lot of work. I can see the rationale for picking Python 3 for new projects, but I don't know if it's worthwhile in this case.