Replace "vobject" library

[raphaelm]

We use "vobject" to write icalendar files. It is unmaintained and inefficient for large files, we should at some point replace it.

[raphaelm]

One example for the inefficiency is here: 9d6f117

[wiffbi]

Do you have any alternatives in mind? Something like https://pypi.org/project/icalendar/ ?

[raphaelm]

IIRC we moved from icalender to vobject years ago, but I don't remember why. I think something was buggy.

I have a hunch that we might need to roll our own.

[wiffbi]

Not sure about this one, but from a first look I like the API: https://github.com/ics-py/ics-py
I do not know, how „solid“ this is, but maybe just as inspiration for our own class-API.