assertItemsEqual was removed from Python's unittest.TestCase in
Python 3.3 [1][2]. We have been able to use them since then, because
testtools required unittest2, which still included it. With testtools
removing Python 2.7 support [3][4], we will lose support for
assertItemsEqual, so we should switch to use assertCountEqual.
[1] - https://bugs.python.org/issue17866
[2] - https://hg.python.org/cpython/rev/d9921cb6e3cd
[3] - testing-cabal/testtools#286
[4] - testing-cabal/testtools#277
Conflicts:
openstackclient/tests/unit/volume/v2/test_volume_snapshot.py
NOTE(jpena): This file is not present in stable/wallaby
Change-Id: I0bbffbec8889b8b3067cfe17d258f5cb16624f38
(cherry picked from commit e82a05864f)
53f1efa010