[py3] Used compatible imports of StringIO.

tests/regressiontests/createsuperuser/tests.py

@@ -1,9 +1,8 @@
-from StringIO import StringIO
-
 from django.contrib.auth import models
 from django.contrib.auth.management.commands import changepassword
 from django.core.management import call_command
 from django.test import TestCase
+from django.utils.six import StringIO
 
 
 class MultiDBChangepasswordManagementCommandTestCase(TestCase):