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.

computes/views.py

@@ -8,10 +8,6 @@ from django.utils import timezone
 from instances.models import Instance
 from libvirt import libvirtError
 from vrtManager.connection import (
-    CONN_SOCKET,
-    CONN_SSH,
-    CONN_TCP,
-    CONN_TLS,
     connection_manager,
     wvmConnect,
 )