Replace various Incompletes in stdlib (#11673)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

stdlib/multiprocessing/util.pyi

@@ -2,7 +2,7 @@ import threading
 from _typeshed import ConvertibleToInt, Incomplete, Unused
 from collections.abc import Callable, Iterable, Mapping, MutableMapping, Sequence
 from logging import Logger, _Level as _LoggingLevel
-from typing import Any
+from typing import Any, Generic, TypeVar, overload
 
 __all__ = [
     "sub_debug",
@@ -22,6 +22,9 @@ __all__ = [
     "SUBWARNING",
 ]
 
+_T = TypeVar("_T")
+_R_co = TypeVar("_R_co", default=Any, covariant=True)
+
 NOTSET: int
 SUBDEBUG: int
 DEBUG: int
@@ -42,13 +45,29 @@ def is_abstract_socket_namespace(address: str | bytes | None) -> bool: ...
 abstract_sockets_supported: bool
 
 def get_temp_dir() -> str: ...
-def register_after_fork(obj, func: Callable[[Incomplete], object]) -> None: ...
+def register_after_fork(obj: _T, func: Callable[[_T], object]) -> None: ...
 
-class Finalize:
+class Finalize(Generic[_R_co]):
+    # "args" and "kwargs" are passed as arguments to "callback".
+    @overload
     def __init__(
         self,
-        obj: Incomplete | None,
-        callback: Callable[..., Incomplete],
+        obj: None,
+        callback: Callable[..., _R_co],
+        *,
+        args: Sequence[Any] = (),
+        kwargs: Mapping[str, Any] | None = None,
+        exitpriority: int,
+    ) -> None: ...
+    @overload
+    def __init__(
+        self, obj: None, callback: Callable[..., _R_co], args: Sequence[Any], kwargs: Mapping[str, Any] | None, exitpriority: int
+    ) -> None: ...
+    @overload
+    def __init__(
+        self,
+        obj: Any,
+        callback: Callable[..., _R_co],
         args: Sequence[Any] = (),
         kwargs: Mapping[str, Any] | None = None,
         exitpriority: int | None = None,
@@ -59,7 +78,7 @@ class Finalize:
         _finalizer_registry: MutableMapping[Incomplete, Incomplete] = {},
         sub_debug: Callable[..., object] = ...,
         getpid: Callable[[], int] = ...,
-    ): ...
+    ) -> _R_co: ...
     def cancel(self) -> None: ...
     def still_active(self) -> bool: ...