Move backend to vdir

[geier]

See vdir and vdirsyncer.

[untitaker]

I wonder whether we should just abandon pycarddav and continue development at khard (although the author hasn't shown much activity)

[geier]

I don't have any sentimental issues with abandoning pycarddav, I currently still use it because some of its features are not yet in khard.

[matthiasbeyer]

I currently still use it because some of its features are not yet in khard.

Same for me! Would love to see more work in khard (unfortunately I'm not a python guy, so I guess I cannot help that much, but I'd be happy to test things out!)