multiprocessing.Process.is_alive() incorrect if waitpid() was executed concurrently to it

Bug report
As per title. In my case I triggered the os.waitpid() using psutil library, but easily reproducible using os.waitpid, too.

Reproduction:

Python 3.9.5 (default, Nov 18 2021, 16:00:48) 
[GCC 10.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from multiprocessing import Process
>>> import os
>>> import time
>>> p = Process(target=time.sleep, args=(10,))
>>> p.start(); os.waitpid(p.pid, 0)
(17587, 0)
>>> p.is_alive()
True
>>> p.join() # Returns immediately
>>> 

I understand it's something of a corner case; however, join() behaves correctly here, so we definitely have enough info about whether the process is alive.

By the looks of it, if the process has been already waited on, poll() Popen method in multiprocessing returns None, which is interpreted as the process being still alive.

Your environment

• CPython versions tested on: 3.9.5, but main version seems affected
• Operating system and architecture: x64 Ubuntu Linux

Beginne damit, multiprocessing.Process.is_alive(), den Popen.poll()-Pfad und Process.join() nachzuverfolgen, während du das nebenläufige os.waitpid()-Beispiel unter Linux reproduzierst. Vergleiche, wie jeder Pfad feststellt, dass der Kindprozess beendet wurde, und füge anschließend eine Regressionstestabdeckung hinzu, die zeigt, dass is_alive() false meldet, nachdem bereits auf den Kindprozess gewartet wurde, und dass join() weiterhin sofort zurückkehrt.