[ci] use a pythonic image

inspired by
https://codeberg.org/Codeberg-CI/examples/src/branch/main/Python/.woodpecker/code-quality.yaml

.woodpecker/workflow.yaml

@@ -4,8 +4,15 @@ when:
 
 steps:
   - name: lint-and-test
-    image: debian
+    image: python:3.13-buster
+    environment:
+      PYTHONDEVMODE: 1
+      DJANGO_SETTINGS_MODULE: dx.settings
+      DJANGO_CONFIGURATION: Dev
     commands:
-      - pip install -r requirements/test.txt
+      - python -m venv venv
+      - /bin/bash -c "source venv/bin/activate"
+      - python -m pip install --upgrade pip
+      - python -m pip install -r requirements/test.txt
       - ruff check src
       - python -m pytest