Prepare release 4.0.0

[phil-davis]

sabre/uri major version 3 has a breaking change (which only applies to Windows-style URIs, but it is breaking for an installation that really uses that). So bump the major version here in sabre/xml so that consumers can choose exactly what combination of sabre/xml plus sabre/uri that they use.

It's a bit sad that I released 3.0.0 only a month ago, but the numbers are integers, and integers go on forever - we won't run out!

[codecov[bot]]

Codecov Report

Merging #228 (74b673b) into master (3829dde) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #228   +/-   ##
=========================================
  Coverage     96.68%   96.68%           
  Complexity      116      116           
=========================================
  Files            13       13           
  Lines           453      453           
=========================================
  Hits            438      438           
  Misses           15       15           

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[phil-davis]

lgtm. are there any open issues which can/should be fixed with this new major, since we are already at it?

I don't see anything that can be made ready easily. They all look like discussions from years ago that "someone" (tm) would have to dig into and understand again. And probably many would be enhancements that would be designed in a backward-compatible way anyhow (so would become 4.1 4.2 etc).