Update main.py

Fix issue with `AgentExecutor` not shutting down and/or starting.

learn/generation/langchain/handbook/09-langchain-streaming/main.py

@@ -45,6 +45,9 @@ agent = initialize_agent(
 class AsyncCallbackHandler(AsyncIteratorCallbackHandler):
     content: str = ""
     final_answer: bool = False
+    
+    def __init__(self) -> None:
+        super().__init__()
 
     async def on_llm_new_token(self, token: str, **kwargs: Any) -> None:
         self.content += token
@@ -101,4 +104,4 @@ if __name__ == "__main__":
         host="localhost",
         port=8000,
         reload=True
-    )
+    )