Add @type_check_only to stub-only private classes in stdlib (#14512)

stdlib/_pickle.pyi

@@ -4,6 +4,7 @@ from pickle import PickleBuffer as PickleBuffer
 from typing import Any, Protocol, type_check_only
 from typing_extensions import TypeAlias
 
+@type_check_only
 class _ReadableFileobj(Protocol):
     def read(self, n: int, /) -> bytes: ...
     def readline(self) -> bytes: ...