Fixed #18269 -- Applied unicode_literals for Python 3 compatibility.

Thanks Vinay Sajip for the support of his django3 branch and
Jannis Leidel for the review.

tests/modeltests/str/tests.py

@@ -1,5 +1,5 @@
  # -*- coding: utf-8 -*-
-from __future__ import absolute_import
+from __future__ import absolute_import, unicode_literals
 
 import datetime
 
@@ -19,7 +19,7 @@ class SimpleTests(TestCase):
 
     def test_international(self):
         a = InternationalArticle.objects.create(
-            headline=u'Girl wins €12.500 in lottery',
+            headline='Girl wins €12.500 in lottery',
             pub_date=datetime.datetime(2005, 7, 28)
         )
         # The default str() output will be the UTF-8 encoded output of __unicode__().