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 from css-parser in 3 minutes and 33 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Test
passed #11045
run-appdata

00:00:07

failed #11044
run-tests

00:03:25

 
  Build
skipped #11046
run-build
 
Name Stage Failure
failed
run-tests Test
************* Module gajim.gtk.css_config
gajim/gtk/css_config.py:23:0: C0411: third party import "from gi.repository import Gtk" should be placed before "import css_parser" (wrong-import-order)
gajim/gtk/css_config.py:24:0: C0411: third party import "from gi.repository import Gdk" should be placed before "import css_parser" (wrong-import-order)
gajim/gtk/css_config.py:25:0: C0411: third party import "from gi.repository import Pango" should be placed before "import css_parser" (wrong-import-order)

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Your code has been rated at 10.00/10 (previous run: 10.00/10, -0.00)

ERROR: Job failed: exit status 1