Update app.py

app.py

@@ -6,6 +6,7 @@ from langchain.chains import LLMChain
 from langchain_core.callbacks import StreamingStdOutCallbackHandler
 from langchain.retrievers import TFIDFRetriever
 from langchain.chains import RetrievalQA
+from langchain.memory import ConversationBufferMemory
 
 
 callbacks = [StreamingStdOutCallbackHandler()]