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/middleware.py

@@ -1,4 +1,4 @@
-from .settings import app_settings, get_settings
+from .settings import get_settings
 
 
 class AppSettingsMiddleware(object):

appsettings/migrations/0007_auto_20220905_0918.py

@@ -1,7 +1,6 @@
 # Generated by Django 3.2.13 on 2022-06-30 07:17
 
 from django.db import migrations
-from django.utils.translation import gettext_lazy as _
 
 def update_default_settings(apps, schema_editor):
     setting = apps.get_model("appsettings", "AppSettings")

appsettings/test_appsettings.py

@@ -1,4 +1,4 @@
-from django.test import Client, TestCase
+from django.test import TestCase
 from django.urls import reverse
 
 from appsettings.models import AppSettings

appsettings/tests.py

@@ -1,3 +1,2 @@
-from django.test import TestCase
 
 # Create your tests here.

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 _