fix: icalendar v6 compatibility

[dotlambda]

fixes #144

See https://github.com/collective/icalendar/blob/v6.0.0/CHANGES.rst for the changelog.

[5ila5]

Is there anyway this could be merged in the near future? Our HomeAssisntant integration relies on icalevents, but breaks if people install other integrations requiring the new calendar version.

See:
mampfes/hacs_waste_collection_schedule#2893

[peterbaumert]

@JamieMagee you know when this can be merged?

[BoGnY]

someone can merge this?!?!?!?!?

3 months waiting to merge a fix

[andreasbrett]

Is there anyway this could be merged in the near future? Our HomeAssisntant integration relies on icalevents, but breaks if people install other integrations requiring the new calendar version.

See: mampfes/hacs_waste_collection_schedule#2893

@5ila5
Since icalevents is such a tiny library and all ICS-related providers in Waste Collection Schedule are affected by these delays here: what do you think about packaging a fork (with this PR merged) into WCS? Just as a workaround that could be changed back to the original pypi icalevents once it works with icalendar 6.