updates KW_ONLY sentinel usage in test stubs

based on python/typeshed#5826

Author: tgallant

mypy/plugins/dataclasses.py

@@ -408,7 +408,7 @@ def _is_kw_only_type(self, node: Optional[Type]) -> bool:
         node_type = get_proper_type(node)
         if not isinstance(node_type, Instance):
             return False
-        return node_type.type.fullname == 'dataclasses._KW_ONLY_TYPE'
+        return node_type.type.fullname == 'dataclasses.KW_ONLY'
 
 
 def dataclass_class_maker_callback(ctx: ClassDefContext) -> None:

test-data/unit/lib-stub/dataclasses.pyi

@@ -5,8 +5,7 @@ _T = TypeVar('_T')
 class InitVar(Generic[_T]):
     ...
 
-class _KW_ONLY_TYPE: ...
-KW_ONLY = _KW_ONLY_TYPE
+class KW_ONLY: ...
 
 @overload
 def dataclass(_cls: Type[_T]) -> Type[_T]: ...