Update app.py

added -- from langchain.callbacks.base import BaseCallbackHandler

app.py

@@ -5,6 +5,7 @@ import openai
 from langchain.prompts import ChatPromptTemplate
 from langchain.chat_models import ChatOpenAI
 from langchain.schema import StrOutputParser
+from langchain.callbacks.base import BaseCallbackHandler
 
 #Type Hinting
 from typing import Any, Callable, Dict, List, Literal, Optional, Tuple, Type, Union