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

stdlib/time.pyi

@@ -1,6 +1,6 @@
 import sys
 from _typeshed import structseq
-from typing import Any, Final, Literal, Protocol, final
+from typing import Any, Final, Literal, Protocol, final, type_check_only
 from typing_extensions import TypeAlias
 
 _TimeTuple: TypeAlias = tuple[int, int, int, int, int, int, int, int, int]
@@ -80,6 +80,7 @@ def time() -> float: ...
 if sys.platform != "win32":
     def tzset() -> None: ...  # Unix only
 
+@type_check_only
 class _ClockInfo(Protocol):
     adjustable: bool
     implementation: str