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.

accounts/test_utils.py

@@ -1,6 +1,5 @@
 import base64
 import struct
-from unittest.mock import patch
 
 from django.contrib.auth import get_user_model
 from django.core import mail

accounts/views.py

@@ -20,7 +20,7 @@ from accounts.apps import admin_password_path
 from instances.models import Instance
 
 from accounts.forms import EmailOTPForm, ProfileForm, UserSSHKeyForm
-from accounts.models import *
+from accounts.models import User, UserAttributes, UserInstance, UserSSHKey
 
 from . import forms
 from .utils import get_user_totp_device, send_email_with_otp