Replace assertItemsEqual with assertCountEqual
    
    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