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

Refs #20680.

tests/validators/tests.py

@@ -1,14 +1,14 @@
 # -*- coding: utf-8 -*-
 from __future__ import unicode_literals
 
+from datetime import datetime, timedelta
 import re
 import types
-from datetime import datetime, timedelta
+from unittest import TestCase
 
 from django.core.exceptions import ValidationError
 from django.core.validators import *
 from django.test.utils import str_prefix
-from django.utils.unittest import TestCase
 
 
 NOW = datetime.now()