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

stdlib/asyncio/streams.pyi

@@ -3,7 +3,7 @@ import sys
 from _typeshed import ReadableBuffer, StrPath
 from collections.abc import Awaitable, Callable, Iterable, Sequence, Sized
 from types import ModuleType
-from typing import Any, Protocol, SupportsIndex
+from typing import Any, Protocol, SupportsIndex, type_check_only
 from typing_extensions import Self, TypeAlias
 
 from . import events, protocols, transports
@@ -25,6 +25,7 @@ else:
 
 _ClientConnectedCallback: TypeAlias = Callable[[StreamReader, StreamWriter], Awaitable[None] | None]
 
+@type_check_only
 class _ReaduntilBuffer(ReadableBuffer, Sized, Protocol): ...
 
 if sys.version_info >= (3, 10):