Use _typeshed.IdentityFunction more consistently (#8063)

stubs/six/six/__init__.pyi

@@ -3,6 +3,7 @@ from __future__ import print_function
 import builtins
 import types
 import unittest
+from _typeshed import IdentityFunction
 from builtins import next as next
 from collections.abc import Callable, ItemsView, Iterable, Iterator as _Iterator, KeysView, Mapping, ValuesView
 from functools import wraps as wraps
@@ -83,7 +84,7 @@ def raise_from(value: BaseException | type[BaseException], from_value: BaseExcep
 print_ = print
 
 def with_metaclass(meta: type, *bases: type) -> type: ...
-def add_metaclass(metaclass: type) -> Callable[[_T], _T]: ...
+def add_metaclass(metaclass: type) -> IdentityFunction: ...
 def ensure_binary(s: bytes | str, encoding: str = ..., errors: str = ...) -> bytes: ...
 def ensure_str(s: bytes | str, encoding: str = ..., errors: str = ...) -> str: ...
 def ensure_text(s: bytes | str, encoding: str = ..., errors: str = ...) -> str: ...