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

Refs #20680.

tests/utils_tests/test_html.py

@@ -2,11 +2,11 @@ from __future__ import unicode_literals
 
 from datetime import datetime
 import os
+from unittest import TestCase
 
 from django.utils import html
 from django.utils._os import upath
 from django.utils.encoding import force_text
-from django.utils.unittest import TestCase
 
 
 class TestUtilsHtml(TestCase):