Refs #23919 -- Removed most of remaining six usage

Thanks Tim Graham for the review.

tests/admin_views/admin.py

@@ -1,5 +1,6 @@
 import os
 import tempfile
+from io import StringIO
 from wsgiref.util import FileWrapper
 
 from django import forms
@@ -17,7 +18,6 @@ from django.forms.models import BaseModelFormSet
 from django.http import HttpResponse, StreamingHttpResponse
 from django.utils.html import format_html
 from django.utils.safestring import mark_safe
-from django.utils.six import StringIO
 
 from .forms import MediaActionForm
 from .models import (