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

Change-Id: I6ebd93aa093caad6f2d3db6e43beb8bc8d9cedb7

yaql/tests/test_strings.py

@@ -174,7 +174,7 @@ class TestStrings(yaql.tests.TestCase):
         self.assertEqual(['a', 'b', 'c'], self.eval('abc.toCharArray()'))
 
     def test_characters(self):
-        self.assertItemsEqual(
+        self.assertCountEqual(
             ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9'],
             self.eval('characters(octdigits => true, digits => true)'))