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.
Status Job ID Name Coverage
  Test
passed #10159
run-appdata

00:00:09

passed #10158
run-tests

00:03:11

canceled #10156
run-appdata

canceled #10155
run-tests

 
  Build
passed #10160
run-build

00:00:08

canceled #10157
run-build