Depend on deprecated vobject

[ArtemIsmagilov]

Why do we need a dependency on vobject? icalendar is currently well supported. It seems that we have the same functionality implemented by different libraries. besides, vobject is not supported.

[tobixen]

For backward compatibility we need to support the .vobject_instance property.

My intention is to look through the library and make sure there are no internal calls to .vobject_instance, move the import of vobject to the vobject_instance-getter and make it into an optional dependency.

[dotlambda]

caldav no longer depends on vobject

[tobixen]

Right, I will be closing this ticket issue (edited).