Update app.py

app.py

@@ -17,6 +17,7 @@ from langchain.schema import (
     SystemMessage
 )
 
+from langchain.chains import ConversationalRetrievalChain
 from langchain.chains.llm import LLMChain
 from langchain.callbacks.base import CallbackManager
 from langchain.callbacks.streaming_stdout import StreamingStdOutCallbackHandler