Bump icalendar from 6.1.0 to 6.3.1

Bumps [icalendar](https://github.com/collective/icalendar) from 6.1.0 to 6.3.1.
- [Release notes](https://github.com/collective/icalendar/releases)
- [Changelog](https://github.com/collective/icalendar/blob/main/CHANGES.rst)
- [Commits](https://github.com/collective/icalendar/compare/v6.1.0...v6.3.1)

---
updated-dependencies:
- dependency-name: icalendar
  dependency-version: 6.3.1
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -20,7 +20,7 @@ requests==2.32.4
 six==1.16.0
 webdavclient3==3.14.6
 whitenoise==6.8.2
-icalendar==6.1.0
+icalendar==6.3.1
 pyyaml==6.0.2
 uritemplate==4.1.1
 beautifulsoup4==4.12.3