Change default child watcher to `PidfdChildWatcher` on supported Linux versions

[kumaraditya303]

PidfdChildWatcher based on pidfd_open is the best child watcher implementation for Linux as it avoids race conditions and is most reliable of all of them. It requires Linux 5.3+ so this will be detected at runtime if it can be used and if so use this instead of ThreadedChildWatcher. asyncio will favor using PidfdChildWatcher over ThreadedChildWatcher in 3.12+ on systems which supports it.

[gvanrossum]

Thanks!

There's a little bit of possible cleanup -- the check whether os.pidfd_open() exists and works (can_use_pidfd()) is duplicated in Lib/test/test_asyncio/test_subprocess.py (as _has_pidfd_support). While can_use_pidfd() is not a documented or public API I think it's better to call it from the test code so there's only one copy to update if the check ever changes.