Add multiprocessing.popen_forkserver submodule (#8425)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

stdlib/multiprocessing/popen_forkserver.pyi

@@ -0,0 +1,22 @@
+import sys
+from multiprocessing.process import BaseProcess
+from typing import ClassVar
+
+from . import popen_fork
+from .util import Finalize
+
+if sys.platform != "win32":
+    __all__ = ["Popen"]
+
+    class _DupFd:
+        def __init__(self, ind: int) -> None: ...
+        def detach(self) -> int: ...
+
+    class Popen(popen_fork.Popen):
+        DupFd: ClassVar[type[_DupFd]]
+        finalizer: Finalize
+        sentinel: int
+
+        def __init__(self, process_obj: BaseProcess) -> None: ...
+        def duplicate_for_child(self, fd: int) -> int: ...
+        def poll(self, flag: int = ...) -> int | None: ...

tests/stubtest_allowlists/win32.txt

@@ -56,8 +56,9 @@ syslog
 termios
 xxlimited
 
-# Multiprocessing.popen_fork exists on Windows but fails to import
+# multiprocessing.popen_fork and popen_forkserver exist on Windows but fail to import
 multiprocessing.popen_fork
+multiprocessing.popen_forkserver
 
 # Modules that rely on _curses
 curses