Revert "assertItemsEqual() moved to assertCountEqual()"

This reverts commit 3d2617983b.

The change was not quite right because the method six provides needs to
have `self` be passed in as well.  See the docs here:
https://six.readthedocs.io/#unittest-assertions

Reverting for now and we can fix it in smaller chunks later.

lms/djangoapps/instructor_task/tests/test_base.py

@@ -8,7 +8,6 @@ import json
 # pylint: disable=attribute-defined-outside-init
 import os
 import shutil
-import six
 from tempfile import mkdtemp
 from uuid import uuid4
 
@@ -370,8 +369,8 @@ class TestReportMixin(object):
                 self.assertEqual(csv_rows, expected_rows)
                 self.assertEqual(numeric_csv_rows, numeric_expected_rows)
             else:
-                six.assertCountEqual(csv_rows, expected_rows)
-                six.assertCountEqual(numeric_csv_rows, numeric_expected_rows)
+                self.assertItemsEqual(csv_rows, expected_rows)
+                self.assertItemsEqual(numeric_csv_rows, numeric_expected_rows)
 
     @staticmethod
     def _extract_and_round_numeric_items(dictionary):