Use timezone.utc for compatibility with Py<3.11

caldav/objects.py

@@ -15,7 +15,6 @@ class hierarchy into a separate file)
 from datetime import datetime
 from datetime import timedelta
 from datetime import timezone
-from datetime import UTC
 from typing import Any
 from typing import List
 from typing import Optional
@@ -3000,7 +2999,7 @@ def complete(
            * safe - see doc for _complete_recurring_safe for details
         """
         if not completion_timestamp:
-            completion_timestamp = datetime.now(UTC)
+            completion_timestamp = datetime.now(timezone.utc)
 
         if "RRULE" in self.icalendar_component and handle_rrule:
             return getattr(self, "_complete_recurring_%s" % rrule_mode)(