ci: use a build matrix instead of anchors for parallel jobs.

Rather than using anchors and references to duplicate job details, use the matrix feature to expand the jobs. This MR keeps the same jobs as the existing pipeline, but this would make it trivial to e.g. test GTK 3 and 4 on every Python version.

I've also cleaned up a few obsolete pieces of gitlab-ci syntax: defaults moved from toplevel to default section, and when replaced with rules:if.