move byref-related definitions from ctypes/__init__.pyi to _ctypes.pyi (#10137)

stdlib/_ctypes.pyi

@@ -2,7 +2,7 @@ import sys
 from _typeshed import ReadableBuffer, WriteableBuffer
 from abc import abstractmethod
 from collections.abc import Iterable, Iterator, Mapping, Sequence
-from ctypes import CDLL, _CArgObject
+from ctypes import CDLL
 from typing import Any, Generic, TypeVar, overload
 from typing_extensions import Self, TypeAlias
 
@@ -89,6 +89,10 @@ class _Pointer(Generic[_CT], _PointerLike, _CData):
 def POINTER(type: type[_CT]) -> type[_Pointer[_CT]]: ...
 def pointer(__arg: _CT) -> _Pointer[_CT]: ...
 
+class _CArgObject: ...
+
+def byref(obj: _CData, offset: int = ...) -> _CArgObject: ...
+
 class _CField:
     offset: int
     size: int