add typing._Final (#13015)

This is the subset of typing module internal base classes
that are stable over all supported versions of python.

stdlib/typing.pyi

@@ -133,6 +133,8 @@ if sys.version_info >= (3, 13):
 
 Any = object()
 
+class _Final: ...
+
 def final(f: _T) -> _T: ...
 @final
 class TypeVar:
@@ -191,7 +193,7 @@ _promote = object()
 
 # N.B. Keep this definition in sync with typing_extensions._SpecialForm
 @final
-class _SpecialForm:
+class _SpecialForm(_Final):
     def __getitem__(self, parameters: Any) -> object: ...
     if sys.version_info >= (3, 10):
         def __or__(self, other: Any) -> _SpecialForm: ...
@@ -973,7 +975,7 @@ class _TypedDict(Mapping[str, object], metaclass=ABCMeta):
         def __ior__(self, value: typing_extensions.Self, /) -> typing_extensions.Self: ...  # type: ignore[misc]
 
 @final
-class ForwardRef:
+class ForwardRef(_Final):
     __forward_arg__: str
     __forward_code__: CodeType
     __forward_evaluated__: bool