Use external library for vobject parsing

[untitaker]

Our current sloppy parser is not exactly a maintenance burden, but it's annoying to have it here in the first place.

Two things need to be resolved for this:

• vcard support as discussed in Roadmap for 4.0 collective/icalendar#186 -- we absolutely need some sort of support because of Problem syncing vcards with group construct #89, Singlefile storage corrupts vcards #70, etc
• lazy parsing, see Lazy parsing collective/icalendar#158 -- eager parsing as of right now is IMO quite slow, and not appropriate given how few operations vdirsyncer actually does on item contents.

I'm unsure if this is worth it, given how well the sloppy parser in vdirsyncer.utils.vobject has worked in the past.

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[WhyNotHugo]

From what I can tell:

cons

Might raise issues due to bugs in collective/icalendar
Code migration cost
Bugfixes might be tied to collective/icalendar releases

pros

Will help find bugs in collective/icalendar
Reduced maintenance cost
Will probably raise awareness of issues we haven't found yet
Maybe we can detect earlier if we're sending invalid events to a server?

[untitaker]

Maybe we can detect earlier if we're sending invalid events to a server?

Interesting idea, but I'm unsure about this, as a "generic" implementation would also prevent downloading broken events from a server. Perhaps put it into the repair command.

[untitaker]

Alternatively use https://github.com/eventable/vobject, which seems to be Py3 compatible and also can parse both iCalendar and Vobject

[untitaker]

Quick note: In #428 we figured out that vobject has many bugs. We do need absolutely correct serialization/deserialization roundtrip because of the singlefile/http storages.

[untitaker]

I think we should use vobject anyway. I wonder if there's a good set of weird vcards somewhere, to test with vobject.

[untitaker]

skarim/vobject#52