[issue38692] add a pidfd child process watcher

[STINNER Victor]

STINNER Victor <vstinner at python.org> added the comment:

> BTW my kernel is: 5.3.7-301.fc31.x86_64

I get a different error on Fedora 31 with Linux kernel 5.3.9-300.fc31.x86_64:

$ ./python
Python 3.9.0a1+ (heads/method_freelist:e34fa9b8d7, Nov 20 2019, 12:09:54) 
[GCC 9.2.1 20190827 (Red Hat 9.2.1-1)] on linux
>>> import os
>>> os.pidfd_open(-1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OSError: [Errno 22] Invalid argument

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<https://bugs.python.org/issue38692>
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