Refs #23919 -- Removed six.<various>_types usage

Thanks Tim Graham and Simon Charette for the reviews.

tests/custom_columns/tests.py

@@ -1,6 +1,5 @@
 from django.core.exceptions import FieldError
 from django.test import TestCase
-from django.utils import six
 
 from .models import Article, Author
 
@@ -20,7 +19,7 @@ class CustomColumnsTests(TestCase):
             Author.objects.all(), [
                 "Peter Jones", "John Smith",
             ],
-            six.text_type
+            str
         )
 
     def test_get_first_name(self):
@@ -34,7 +33,7 @@ class CustomColumnsTests(TestCase):
             Author.objects.filter(first_name__exact="John"), [
                 "John Smith",
             ],
-            six.text_type
+            str
         )
 
     def test_field_error(self):
@@ -54,7 +53,7 @@ class CustomColumnsTests(TestCase):
                 "Peter Jones",
                 "John Smith",
             ],
-            six.text_type
+            str
         )
 
     def test_get_all_articles_for_an_author(self):
@@ -70,7 +69,7 @@ class CustomColumnsTests(TestCase):
             self.article.authors.filter(last_name='Jones'), [
                 "Peter Jones"
             ],
-            six.text_type
+            str
         )
 
     def test_author_querying(self):