Add missing multiprocessing.popen_fork type stubs

[kkirsche]

X-Ref: #6799

This adds best-effort types to multiprocessing.popen_fork to continue to address issue #6799

The source file can be viewed here:
https://github.com/python/cpython/blob/main/Lib/multiprocessing/popen_fork.py

Popen attributes in some cases may not exist, such as sentinel which is set here but not initialized during the __init__. As such, it may desirable for those attributes to be removed from the stub.

[AlexWaygood]

Since the module fails to import on Windows, you'll need to add an entry to the stubtest allowlist here:

typeshed/tests/stubtest_allowlists/win32.txt

Lines 39 to 57 in 804628d

	# Modules that do not exist on Windows systems
	_curses
	_posixsubprocess
	asyncio.unix_events
	crypt
	dbm.gnu
	dbm.ndbm
	fcntl
	grp
	nis
	ossaudiodev
	posix
	pwd
	readline
	resource
	spwd
	syslog
	termios
	xxlimited

(Add a comment explaining that it does "exist" on Windows, but it fails to import.)

[AlexWaygood]

Since the module fails to import on Windows

You should also put the entire module contents behind an if sys.platform != "win32" guard, like we do in e.g. stdlib/_curses.pyi.

[AlexWaygood]

Mostly looks good

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉