gh-83069: Use efficient event-driven subprocess.Popen.wait() on Linux / macOS / BSD

[giampaolo]

This patch replaces the busy-loop mechanism used by subprocess.Popen.wait() on Linux, BSD, and macOS when a timeout is specified:

• Linux ≥= 5.3: uses os.pidfd_open() + select.poll()
• macOS, BSD: uses select.kqueue() + KQ_FILTER_PROC + KQ_NOTE_EXIT

The kernel wakes us up exactly when the PID terminates or the timeout expires. If the fast-wait mechanisms is not available or fails (e.g. "too many open files"), the function falls back to the traditional busy loop. A similar approach was recently implemented in psutil (see giampaolo/psutil#2706). While working on that change, I noticed that the subprocess module used essentially the same busy-loop strategy, which motivated this proposal.

• Issue: Popen should use pidfd_open to implement a non-busy wait #83069

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