With the removal of Python 2.x we can remove the unittest2 compat
wrappers and switch to assertCountEqual instead of assertItemsEqual
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^
Change-Id: Ied2227a482087f4a2dc4e2d9986f9b3b777aa821
4d58c0bb3d