Fix a missing import

Author: seratch

slack_sdk/web/async_internal_utils.py

@@ -3,7 +3,7 @@
 import logging
 from asyncio import AbstractEventLoop
 from logging import Logger
-from typing import Optional, BinaryIO, Dict, Sequence
+from typing import Optional, BinaryIO, Dict, Sequence, Union
 
 import aiohttp
 from aiohttp import ClientSession