Migrate to python3 + cleanup

[calixteman]

Fixes #282.

[coveralls]

Coverage decreased (-4.5%) to 25.985% when pulling 3eab2a2 on calixteman:python3 into 792aede on mozilla:master.

ci errors

[sylvestre]

      'f3': utils.get_flag(self.versions['release'], 'tracking', 'release'),
   TypeError: 'NoneType' object is not subscriptable

[sylvestre]

please fix that

[marco-c]

The error is in auto_nag/scripts/tracked_bad_severity.py:

   Traceback (most recent call last):
    auto_nag/tests/test_bzcleaner.py line 51 in test_get_bz_params
      p = TrackedBadSeverity().get_bz_params(None)
    auto_nag/scripts/tracked_bad_severity.py line 31 in get_bz_params
      'f3': utils.get_flag(self.versions['release'], 'tracking', 'release'),

[calixteman]

The bug is because of non-following versions in p-d when the test ran yesterday. I'm fixing it.