Add type hints for timezone.py

src/linkahead/common/timezone.py

+from __future__ import annotations
+from typing import Optional
+
+
 class TimeZone():
     """
     TimeZone, e.g. CEST, Europe/Berlin, UTC+4.
@@ -13,7 +17,7 @@ class TimeZone():
         A human-friendly name of the time zone:
     """
 
-    def __init__(self, zone_id: str, offset: int, display_name: str):
+    def __init__(self, zone_id: Optional[str], offset: Optional[int], display_name: str):
         self.zone_id = zone_id
         self.offset = offset
         self.display_name = display_name