Refs #23919 -- Removed most of remaining six usage

Thanks Tim Graham for the review.

tests/test_runner/test_debug_sql.py

@@ -1,10 +1,10 @@
 import sys
 import unittest
+from io import StringIO
 
 from django.db import connection
 from django.test import TestCase
 from django.test.runner import DiscoverRunner
-from django.utils import six
 
 from .models import Person
 
@@ -33,7 +33,7 @@ class TestDebugSQL(unittest.TestCase):
         suite.addTest(self.ErrorTest())
         suite.addTest(self.PassingTest())
         old_config = runner.setup_databases()
-        stream = six.StringIO()
+        stream = StringIO()
         resultclass = runner.get_resultclass()
         runner.test_runner(
             verbosity=verbosity,