ci: replace super-linter with ruff

Super-linter ran from the unpinned :latest image and, because of
branches-ignore: [master], never on pull requests to master. Ruff now
checks syntax errors and pyflakes rules (E9, F) on every push and PR.

Fixes the existing findings: unused imports and variables, an unused
duplicate import in instances/tests.py, and the star import in
accounts/views.py. settings-dev.py keeps its star import of the base
settings.

appsettings/views.py

@@ -3,7 +3,6 @@ import os
 import sass
 from admin.decorators import superuser_only
 from django.contrib import messages
-from django.contrib.auth.decorators import login_required
 from django.http import HttpResponseRedirect
 from django.shortcuts import render
 from django.utils.translation import gettext_noop as _