CI: Sync list of dependencies with README

This adds python3-gi-cairo (which does not seem to be actually needed
for running the test suite, so it did not fail without his, but better
to keep the list in sync with the README anyway). It also reorders the
list slightly, to match the README order.

.github/workflows/testing.yml

@@ -38,7 +38,7 @@ jobs:
           # Install a specific python version as configured
           PACKAGES="$PACKAGES python${{matrix.python-version}}"
           # Normal dependencies
-          PACKAGES="$PACKAGES gettext intltool python3-gi python3-cairo python3-dbus python3-xdg libglib2.0-dev libglib2.0-bin gir1.2-gtk-3.0 gtk-update-icon-cache python3-distutils"
+          PACKAGES="$PACKAGES gettext intltool python3-gi python3-cairo python3-gi-cairo python3-distutils python3-dbus python3-xdg libglib2.0-dev libglib2.0-bin gir1.2-gtk-3.0 gtk-update-icon-cache"
           # For dbus-launch
           PACKAGES="$PACKAGES dbus-x11"