Fix missing Python import for List

Changelog: fixed

src/mattermost/__main__.py

@@ -3,7 +3,7 @@
 # SPDX-License-Identifier: MIT
 import os
 import re
-from typing import Union
+from typing import List, Union
 
 from dotenv import load_dotenv
 from helmholtz_cloud_agent.core.main import HCAApplication