Update app.py

app.py

@@ -637,6 +637,7 @@ from langchain.chains import RetrievalQA
 from langchain.chains.conversation.memory import ConversationBufferWindowMemory
 from langchain.agents import Tool, initialize_agent
 from huggingface_hub import login
+from typing import Optional
 
 from pydub import AudioSegment
 import io