Fixed #23930 -- Added copies of captured_std* managers from CPython's test.support.

StringIO import was adapted for compatibility with Python 2.

tests/servers/test_basehttp.py

@@ -1,10 +1,9 @@
-import sys
-
 from django.core.handlers.wsgi import WSGIRequest
 from django.core.servers.basehttp import WSGIRequestHandler
 from django.test import TestCase
 from django.test.client import RequestFactory
-from django.utils.six import BytesIO, StringIO
+from django.test.utils import captured_stderr
+from django.utils.six import BytesIO
 
 
 class WSGIRequestHandlerTestCase(TestCase):
@@ -14,14 +13,10 @@ class WSGIRequestHandlerTestCase(TestCase):
 
         handler = WSGIRequestHandler(request, '192.168.0.2', None)
 
-        _stderr = sys.stderr
-        sys.stderr = StringIO()
-        try:
+        with captured_stderr() as stderr:
             handler.log_message("GET %s %s", str('\x16\x03'), "4")
             self.assertIn(
                 "You're accessing the developement server over HTTPS, "
                 "but it only supports HTTP.",
-                sys.stderr.getvalue()
+                stderr.getvalue()
             )
-        finally:
-            sys.stderr = _stderr