Update to be compatible with python 3.6 and 3.7

app.py

@@ -3,7 +3,11 @@ import logging.handlers
 import queue
 import urllib.request
 from pathlib import Path
-from typing import Literal
+
+try:
+    from typing import Literal
+except ImportError:
+    from typing_extensions import Literal  # type: ignore
 
 import av
 import cv2