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

Refs #20680.

tests/validation/test_error_messages.py

@@ -1,10 +1,11 @@
 # -*- encoding: utf-8 -*-
 from __future__ import unicode_literals
 
+from unittest import TestCase
+
 from django.core.exceptions import ValidationError
 from django.db import models
 from django.utils import six
-from django.utils.unittest import TestCase
 
 
 class ValidationMessagesTest(TestCase):