Redefined dataclasses.KW_ONLY (#5826)

Redefined dataclasses.KW_ONLY so it's a type alias rather than a class instance. Class instances are illegal to use within a type annotation.

stdlib/dataclasses.pyi

@@ -13,8 +13,7 @@ class _MISSING_TYPE: ...
 MISSING: _MISSING_TYPE
 
 if sys.version_info >= (3, 10):
-    class _KW_ONLY_TYPE: ...
-    KW_ONLY: _KW_ONLY_TYPE
+    class KW_ONLY: ...
 
 @overload
 def asdict(obj: Any) -> Dict[str, Any]: ...