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.

.github/workflows/linter.yml

@@ -1,92 +1,27 @@
-###########################
-###########################
-## Linter GitHub Actions ##
-###########################
-###########################
+name: Lint
 
-name: Lint Code Base
-
-#
-# Documentation:
-# https://help.github.com/en/articles/workflow-syntax-for-github-actions
-#
-
-#############################
-# Start the job on all push #
-#############################
 on:
   push:
     branches: [master]
   pull_request:
-    branches-ignore: [master]
-    
+    branches: [master]
 
-###############
-# Set the Job #
-###############
 jobs:
-  build:
-    name: Lint Code Base
-    # Set the agent to run on
+  ruff:
+    name: Ruff
     runs-on: ubuntu-latest
-
-    ##################
-    # Load all steps #
-    ###################
     steps:
-      ##########################
-      # Checkout the code base #
-      ##########################
-    - name: Checkout Code
-      uses: actions/checkout@v4
-      with:
-          # Full git history is needed to get a proper list of changed files within `super-linter`
-          fetch-depth: 0
+      - name: Checkout Code
+        uses: actions/checkout@v4
 
-    - name: Set up Python
-      uses: actions/setup-python@v5
-      with:
-        python-version: '3.12'
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.12'
 
-    - name: Install Required packages
-      run: |
-        sudo apt-get update -qq
-        sudo apt-get install -y python3-venv python3-virtualenv libvirt-dev libldap2-dev libsasl2-dev python3-lxml zlib1g-dev libxslt1-dev pkg-config gcc libssl-dev
-
-    - name: Create & Activate VENV
-      run: |
-        python3 -m venv venv
-        echo "$GITHUB_WORKSPACE/venv/bin" >> "$GITHUB_PATH"
-
-    - name: Install dependencies
-      run: |
-        pip3 install --upgrade pip setuptools wheel
-        if [ -f dev/requirements.txt ]; then pip3 install -r dev/requirements.txt; else pip3 install -r conf/requirements.txt; fi
-    ################################
-    # Run Linter against code base #
-    ################################
-    - name: Lint Code Base
-      uses: docker://github/super-linter:latest
-      env:
-        FILTER_REGEX_EXCLUDE: .*(static|scss|venv|locale|templates)/.*
-        DEFAULT_BRANCH: master
-        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-        VALIDATE_ALL_CODEBASE: false
-        VALIDATE_MARKDOWN: false
-        VALIDATE_HTML: false
-        VALIDATE_JSCPD: false
-        VALIDATE_ANSIBLE: false
-        VALIDATE_CLOJURE: false
-        VALIDATE_COFFEE: false
-        VALIDATE_DART: false
-        VALIDATE_GO: false
-        VALIDATE_JSX: false
-        VALIDATE_KOTLIN: false
-        VALIDATE_POWERSHELL: false
-        VALIDATE_PERL: false
-        VALIDATE_PHP: false
-        VALIDATE_RAKU: false
-        VALIDATE_RUBY: false
-        VALIDATE_TSX: false
-        VALIDATE_TERRAFORM: false
+      # Same ruff version as dev/requirements.txt
+      - name: Install Ruff
+        run: pip install "$(grep '^ruff==' dev/requirements.txt)"
 
+      - name: Run Ruff
+        run: ruff check --output-format=github .