Support sabre/xml v4

[gharlan]

I would like to use sabre/vobject together with latest sabre/xml v4.
Latest vobject release still uses sabre/xml v2.

[codecov]

Codecov Report

Merging #603 (21918bf) into master (041ceaf) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #603   +/-   ##
=========================================
  Coverage     98.76%   98.76%           
  Complexity     1865     1865           
=========================================
  Files            71       71           
  Lines          5325     5325           
=========================================
  Hits           5259     5259           
  Misses           66       66           

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[staabm]

lgtm. the main diff of 4.x was the sabre/uri difference described in https://github.com/sabre-io/xml/releases/tag/4.0.0

I think we only need to make sure, when we release this change that we also note about this sabre/uri implication in the releases notes.

the failling builds are interessting though

[phil-davis]

I restrted the failed jobs, and they failed again, so there is "something special" that needs looking into.

[phil-davis]

See #604 - there are new problems in CI that are unrelated to this PR.
I will see if they can be resolved first, before making the change to sabre/xml v4.

[phil-davis]

https://github.com/sabre-io/vobject/releases/tag/4.5.2 is available.