Add ``icalendar`` requirement to the build.
Add ``icalendar`` requirement to the build.

https://github.com/madewithbytes/us_ignite/issues/106

This package allows to generate ics calendar events.

us-ignite/us_ignite,us-ignite/us_ignite,us-ignite/us_ignite,us-ignite/us_ignite,us-ignite/us_ignite