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
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>>> 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

Linux で並行な os.waitpid() の例を再現しながら、まず multiprocessing.Process.is_alive()、Popen.poll() の経路、Process.join() を追跡します。各経路が子プロセスの終了をどのように判定するかを比較し、その後、子プロセスがすでに待機された後は is_alive() が false を報告し、join() が引き続き即時に完了することを示すリグレッションテストを追加します。