Stopped using django.utils.unittest in the test suite.

Refs #20680.

tests/str/tests.py

@@ -2,10 +2,10 @@
 from __future__ import absolute_import, unicode_literals
 
 import datetime
+from unittest import skipIf
 
 from django.test import TestCase
 from django.utils import six
-from django.utils.unittest import skipIf
 
 from .models import Article, BrokenArticle, InternationalArticle