[multiprocessing] Clarify incomplete popen types

stdlib/multiprocessing/context.pyi

@@ -201,6 +201,13 @@ if sys.platform != "win32":
         Process: ClassVar[type[ForkServerProcess]]
 
 def _force_start_method(method: str) -> None: ...
-def get_spawning_popen() -> Any | None: ...
-def set_spawning_popen(popen: Any) -> None: ...
+
+if sys.platform != "win32":
+    def get_spawning_popen() -> popen_forkserver.Popen | popen_spawn_posix.Popen | None: ...
+    def set_spawning_popen(popen: popen_forkserver.Popen | popen_spawn_posix.Popen | None) -> None: ...
+
+else:
+    def get_spawning_popen() -> popen_spawn_win32.Popen | None: ...
+    def set_spawning_popen(popen: popen_spawn_win32.Popen | None) -> None: ...
+
 def assert_spawning(obj: Any) -> None: ...

stdlib/multiprocessing/heap.pyi

@@ -1,7 +1,7 @@
 import sys
-from _typeshed import Incomplete
 from collections.abc import Callable
 from mmap import mmap
+from multiprocessing import popen_forkserver, popen_spawn_posix, resource_sharer
 from typing import Protocol, type_check_only
 from typing_extensions import TypeAlias
 
@@ -24,7 +24,12 @@ if sys.platform != "win32":
     class _SupportsDetach(Protocol):
         def detach(self) -> int: ...
 
-    def reduce_arena(a: Arena) -> tuple[Callable[[int, _SupportsDetach], Arena], tuple[int, Incomplete]]: ...
+    def reduce_arena(
+        a: Arena,
+    ) -> tuple[
+        Callable[[int, _SupportsDetach], Arena],
+        tuple[int, popen_forkserver._DupFd | popen_spawn_posix._DupFd | resource_sharer.DupFd],
+    ]: ...
     def rebuild_arena(size: int, dupfd: _SupportsDetach) -> Arena: ...
 
 class Heap:

stdlib/multiprocessing/reduction.pyi

@@ -6,7 +6,7 @@ from abc import ABCMeta
 from builtins import type as Type  # alias to avoid name clash
 from collections.abc import Callable
 from copyreg import _DispatchTableType
-from multiprocessing import connection
+from multiprocessing import connection, popen_forkserver, popen_spawn_posix, resource_sharer
 from socket import socket
 from typing import Any, Final
 
@@ -50,7 +50,7 @@ else:
     def send_handle(conn: HasFileno, handle: int, destination_pid: Unused) -> None: ...
     def recv_handle(conn: HasFileno) -> int: ...
     def sendfds(sock: socket, fds: list[int]) -> None: ...
-    def DupFd(fd: int) -> Any: ...  # Return type is really hard to get right
+    def DupFd(fd: int) -> popen_forkserver._DupFd | popen_spawn_posix._DupFd | resource_sharer.DupFd: ...
 
 # These aliases are to work around pyright complaints.
 # Pyright doesn't like it when a class object is defined as an alias