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

Inizia tracciando multiprocessing.Process.is_alive(), il percorso di Popen.poll() e Process.join() mentre riproduci l’esempio concorrente di os.waitpid() su Linux. Confronta il modo in cui ciascun percorso determina che il processo figlio è terminato, quindi aggiungi una copertura di regressione che dimostri che is_alive() restituisce false dopo che si è già atteso il processo figlio e che join() rimane immediato.