Add missing multiprocessing.popen_fork type stubs

stdlib/multiprocessing/popen_fork.pyi

@@ -0,0 +1,23 @@
+import sys
+from typing import ClassVar
+
+from .process import BaseProcess
+from .util import Finalize
+
+if sys.platform != "win32":
+    __all__ = ["Popen"]
+
+    class Popen:
+        finalizer: Finalize | None
+        method: ClassVar[str]
+        pid: int
+        returncode: int | None
+        sentinel: int  # doesn't exist if os.fork in _launch returns 0
+
+        def __init__(self, process_obj: BaseProcess) -> None: ...
+        def duplicate_for_child(self, fd: int) -> int: ...
+        def poll(self, flag: int = ...) -> int | None: ...
+        def wait(self, timeout: float | None = ...) -> int | None: ...
+        def terminate(self) -> None: ...
+        def kill(self) -> None: ...
+        def close(self) -> None: ...

tests/stubtest_allowlists/win32.txt

@@ -56,6 +56,9 @@ syslog
 termios
 xxlimited
 
+# Multiprocessing.popen_fork exists on Windows but fails to import
+multiprocessing.popen_fork
+
 # Modules that rely on _curses
 curses
 curses.ascii