fix: add missing multiprocessing.forkserver type hints

stdlib/multiprocessing/forkserver.pyi

@@ -0,0 +1,32 @@
+from _typeshed import FileDescriptorLike
+from collections.abc import Sequence
+from struct import Struct
+from typing import Any
+
+__all__ = ["ensure_running", "get_inherited_fds", "connect_to_new_process", "set_forkserver_preload"]
+
+MAXFDS_TO_SEND: int
+SIGNED_STRUCT: Struct
+
+class ForkServer:
+    def __init__(self) -> None: ...
+    def set_forkserver_preload(self, modules_names: list[str]) -> None: ...
+    def get_inherited_fds(self) -> list[int] | None: ...
+    def connect_to_new_process(self, fds: Sequence[int]) -> tuple[int, int]: ...
+    def ensure_running(self) -> None: ...
+
+def main(
+    listener_fd: int | None,
+    alive_r: FileDescriptorLike,
+    preload: Sequence[str],
+    main_path: str | None = ...,
+    sys_path: object | None = ...,
+) -> None: ...
+def read_signed(fd: int) -> Any: ...
+def write_signed(fd: int, n: int) -> None: ...
+
+_forkserver: ForkServer = ...
+ensure_running = _forkserver.ensure_running
+get_inherited_fds = _forkserver.get_inherited_fds
+connect_to_new_process = _forkserver.connect_to_new_process
+set_forkserver_preload = _forkserver.set_forkserver_preload