Refs #23919 -- Removed most of remaining six usage

Thanks Tim Graham for the review.

tests/fixtures/tests.py

@@ -3,6 +3,7 @@ import sys
 import tempfile
 import unittest
 import warnings
+from io import StringIO
 
 from django.apps import apps
 from django.contrib.sites.models import Site
@@ -15,7 +16,6 @@ from django.db import IntegrityError, connection
 from django.test import (
     TestCase, TransactionTestCase, mock, skipUnlessDBFeature,
 )
-from django.utils import six
 from django.utils.encoding import force_text
 
 from .models import (
@@ -52,7 +52,7 @@ class DumpDataAssertMixin(object):
     def _dumpdata_assert(self, args, output, format='json', filename=None,
                          natural_foreign_keys=False, natural_primary_keys=False,
                          use_base_manager=False, exclude_list=[], primary_keys=''):
-        new_io = six.StringIO()
+        new_io = StringIO()
         if filename:
             filename = os.path.join(tempfile.gettempdir(), filename)
         management.call_command('dumpdata', *args, **{'format': format,
@@ -445,7 +445,7 @@ class FixtureLoadingTests(DumpDataAssertMixin, TestCase):
     def test_dumpdata_with_uuid_pks(self):
         m1 = PrimaryKeyUUIDModel.objects.create()
         m2 = PrimaryKeyUUIDModel.objects.create()
-        output = six.StringIO()
+        output = StringIO()
         management.call_command(
             'dumpdata', 'fixtures.PrimaryKeyUUIDModel', '--pks', ', '.join([str(m1.id), str(m2.id)]),
             stdout=output,
@@ -471,7 +471,7 @@ class FixtureLoadingTests(DumpDataAssertMixin, TestCase):
         stdout is a tty, and verbosity > 0.
         """
         management.call_command('loaddata', 'fixture1.json', verbosity=0)
-        new_io = six.StringIO()
+        new_io = StringIO()
         new_io.isatty = lambda: True
         with NamedTemporaryFile() as file:
             options = {
@@ -485,7 +485,7 @@ class FixtureLoadingTests(DumpDataAssertMixin, TestCase):
 
             # Test no progress bar when verbosity = 0
             options['verbosity'] = 0
-            new_io = six.StringIO()
+            new_io = StringIO()
             new_io.isatty = lambda: True
             options.update({'stdout': new_io, 'stderr': new_io})
             management.call_command('dumpdata', 'fixtures', **options)
@@ -583,7 +583,7 @@ class FixtureLoadingTests(DumpDataAssertMixin, TestCase):
         ])
 
     def test_loaddata_verbosity_three(self):
-        output = six.StringIO()
+        output = StringIO()
         management.call_command('loaddata', 'fixture1.json', verbosity=3, stdout=output, stderr=output)
         command_output = force_text(output.getvalue())
         self.assertIn(
@@ -689,7 +689,7 @@ class NonExistentFixtureTests(TestCase):
     """
 
     def test_loaddata_not_existent_fixture_file(self):
-        stdout_output = six.StringIO()
+        stdout_output = StringIO()
         with self.assertRaisesMessage(CommandError, "No fixture named 'this_fixture_doesnt_exist' found."):
             management.call_command('loaddata', 'this_fixture_doesnt_exist', stdout=stdout_output)