With the removal of Python 2.x we can remove the unittest2 compat
wrappers and switch to assertCountEqual instead of assertItemsEqual.
Change-Id: I682997a4fceb139419a4b12d9c8fc6c504c329d5
We should only be using unittest.mock since we're python3-only,
so add and enable a hacking check that will catch it.
Change-Id: I64847f329ee3e1a3377a8c63df90c697d8b74c86