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: Ic0d8c9e1025cd05f4042f1c5ba440cc03a0b5834
3f004e6a9f