Replace css-utils with css-parser

The latter is a maintained fork that runs natively on python2/python3
(without using the fragile 2to3), fixes deprecations, and has a passing
testsuite.
3 jobs for master in 2 minutes and 47 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #13057
run-appdata

00:00:10

passed #13056
run-tests

00:02:26

 
  Build
passed #13058
run-build

00:00:10