Don't use datetime.utcnow() (deprecated).

caldav/objects.py

@@ -15,6 +15,7 @@ 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
@@ -2999,7 +3000,7 @@ def complete(
            * safe - see doc for _complete_recurring_safe for details
         """
         if not completion_timestamp:
-            completion_timestamp = datetime.utcnow().astimezone(timezone.utc)
+            completion_timestamp = datetime.now(UTC)
 
         if "RRULE" in self.icalendar_component and handle_rrule:
             return getattr(self, "_complete_recurring_%s" % rrule_mode)(