(Merge 3.4) Closes #21582: Cleanup test_asyncore. Patch written by diana.

- Use support.captured_stderr() where appropriate
- Removes some "from test.support import xxx" import and uses support.xxx
  instead.

diff --cc Lib/test/test_asyncore.py
index b04aa1d8be440106c56948bdcaacab0e7b9db48d,7defe65387795a7b090822ab6d71582fb0e854de..ef02c3300026684e3a6d86b2d82eada47ae6a502
--- 1/Lib/test/test_asyncore.py
--- 2/Lib/test/test_asyncore.py
+++ b/Lib/test/test_asyncore.py
@@@ -5,14 -5,12 +5,11 @@@ import o
  import socket
  import sys
  import time
- import warnings
  import errno
  import struct
 -import warnings
  
  from test import support
- from test.support import TESTFN, run_unittest, unlink, HOST, HOSTv6
  from io import BytesIO
- from io import StringIO
  
  try:
      import threading