If this is done, it should probably be on by default on all --with-pydebug buildbots.  Otherwise I suspect nobody will _ever_ look at its output and we should just remove the feature all together.

Being off in the main process on the build bots would still make test_subprocess.py happier though as that test's own child processes wouldn't pass the command line flag to enable it so they wouldn't see it in child process stderr.