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

stdlib/typing_extensions.pyi

@@ -472,6 +472,7 @@ else:
     def is_protocol(tp: type, /) -> bool: ...
     def get_protocol_members(tp: type, /) -> frozenset[str]: ...
     @final
+    @type_check_only
     class _NoDefaultType: ...
 
     NoDefault: _NoDefaultType
@@ -603,6 +604,7 @@ class Doc:
     def __eq__(self, other: object) -> bool: ...
 
 # PEP 728
+@type_check_only
 class _NoExtraItemsType: ...
 
 NoExtraItems: _NoExtraItemsType