The assertItemsEqual method has been removed in Python 3.3 [1] but
it was kept alive by unittest2, imported by testtools.
To prevent mayhem and despair caused by testtools removing unittest2
from its requirements in the near future, we switch to assertCountEqual.
[1] https://bugs.python.org/issue17866
Change-Id: I614bcedc9981370fb54cfe3da997a3d52484c07f
(cherry picked from commit ca1c1b87af)
cfc6818177