Support for non-gregorian calendar scales

[niccokunzmann]

We do not support different calendar scales. However, there is an ICAL spec for this:
https://datatracker.ietf.org/doc/html/draft-ietf-calext-rscale-04
We use dateutil and it is not implemented:
https://dateutil.readthedocs.io/en/stable/rrule.html

TODO:

• evaluate if icalendar needs notification/is implementing the parsing
• notify dateutil
• test examples

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