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.

datasource/test_datasource.py

@@ -1,11 +1,10 @@
 import json
-from unittest.mock import MagicMock, patch
 
 from django.test import RequestFactory, TestCase
 from django.urls import reverse
 
 from computes.models import Compute
-from datasource.views import get_client_ip, get_hostname_by_ip, os_index, os_metadata_json, os_userdata
+from datasource.views import get_client_ip, get_hostname_by_ip
 
 
 class DataSourceTestCase(TestCase):

datasource/tests.py

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