Switch to typing_extensions for TypedDict

Pydantic politely asked me to do it in an error message for serde

langkit/core/workflow.py

@@ -3,10 +3,10 @@
 import time
 from abc import ABC, abstractmethod
 from dataclasses import dataclass
-from typing import Dict, List, Mapping, Optional, Set, Tuple, TypedDict, Union, cast, overload
+from typing import Dict, List, Mapping, Optional, Set, Tuple, Union, cast, overload
 
 import pandas as pd
-from typing_extensions import NotRequired
+from typing_extensions import NotRequired, TypedDict
 
 from langkit.core.context import Context
 from langkit.core.metric import (