loosen version constraint on icalendar to support v5

Author: fphammerle

CHANGELOG.md

@@ -5,6 +5,9 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Added
+- support [python3-icalendar](https://github.com/collective/icalendar/blob/main/CHANGES.rst) v5
+
 ### Fixed
 - `_event_prop_equal`: fix "TypeError: object of type 'vDDDLists' has no len()"