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.

vrtManager/test_xml_safety.py

@@ -290,7 +290,6 @@ class TestPersistentDiskEditing(unittest.TestCase):
         )
 
     def test_running_bus_change_preserves_disk_and_pending_domain_configuration(self):
-        from lxml import etree
         self.assertTrue(self.domain.isActive())
         live_before = self.domain.XMLDesc(0)
         self.edit(bus="sata")