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

stdlib/_io.pyi

@@ -88,6 +88,7 @@ class BytesIO(BufferedIOBase, _BufferedIOBase, BinaryIO):  # type: ignore[misc]
     def readlines(self, size: int | None = None, /) -> list[bytes]: ...
     def seek(self, pos: int, whence: int = 0, /) -> int: ...
 
+@type_check_only
 class _BufferedReaderStream(Protocol):
     def read(self, n: int = ..., /) -> bytes: ...
     # Optional: def readall(self) -> bytes: ...