Bump icalendar from 4.0.3 to 4.0.4

Bumps [icalendar](https://github.com/collective/icalendar) from 4.0.3 to 4.0.4.
- [Release notes](https://github.com/collective/icalendar/releases)
- [Changelog](https://github.com/collective/icalendar/blob/master/CHANGES.rst)
- [Commits](collective/icalendar@4.0.3...4.0.4)

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

requirements.txt

@@ -10,7 +10,7 @@ python-Levenshtein>=0.12.0
 pytz>=2018.9
 whatthepatch>=0.0.5
 icalevents==0.1.23
-icalendar==4.0.3
+icalendar==4.0.4
 sqlalchemy==1.3.10
 filelock==3.0.12
 alembic==1.3.0