_posixsubprocess: improve types (#9009)

Author: JelleZijlstra

stdlib/_posixsubprocess.pyi

@@ -1,24 +1,32 @@
 import sys
+from _typeshed import StrOrBytesPath
 from collections.abc import Callable, Sequence
+from typing_extensions import SupportsIndex
 
 if sys.platform != "win32":
     def cloexec_pipe() -> tuple[int, int]: ...
     def fork_exec(
-        args: Sequence[str],
-        executable_list: Sequence[bytes],
-        close_fds: bool,
-        fds_to_keep: Sequence[int],
-        cwd: str,
-        env_list: Sequence[bytes],
-        p2cread: int,
-        p2cwrite: int,
-        c2pred: int,
-        c2pwrite: int,
-        errread: int,
-        errwrite: int,
-        errpipe_read: int,
-        errpipe_write: int,
-        restore_signals: int,
-        start_new_session: int,
-        preexec_fn: Callable[[], None],
+        __process_args: Sequence[StrOrBytesPath] | None,
+        __executable_list: Sequence[bytes],
+        __close_fds: bool,
+        __fds_to_keep: tuple[int, ...],
+        __cwd_obj: str,
+        __env_list: Sequence[bytes] | None,
+        __p2cread: int,
+        __p2cwrite: int,
+        __c2pred: int,
+        __c2pwrite: int,
+        __errread: int,
+        __errwrite: int,
+        __errpipe_read: int,
+        __errpipe_write: int,
+        __restore_signals: int,
+        __call_setsid: int,
+        __pgid_to_set: int,
+        __gid_object: SupportsIndex | None,
+        __groups_list: list[int] | None,
+        __uid_object: SupportsIndex | None,
+        __child_umask: int,
+        __preexec_fn: Callable[[], None],
+        __allow_vfork: bool,
     ) -> int: ...