Fix CI: use system python3 + apt-get instead of actions/setup-python

.gitea/workflows/ci.yml

@@ -9,29 +9,29 @@ on:
 jobs:
   lint-and-test:
     runs-on: ubuntu-latest
-    container: python:3.11-slim
     defaults:
       run:
         working-directory: ./backend
 
     steps:
-      - name: Install git
-        working-directory: /
-        run: apt-get update && apt-get install -y git
-
       - uses: actions/checkout@v4
 
+      - name: Set up Python
+        run: |
+          apt-get update && apt-get install -y python3 python3-pip python3-venv || true
+          python3 --version
+
       - name: Install dependencies
         run: |
-          python -m pip install --upgrade pip
-          pip install -r requirements.txt
-          pip install -r requirements-dev.txt
+          python3 -m pip install --upgrade pip
+          python3 -m pip install -r requirements.txt
+          python3 -m pip install -r requirements-dev.txt
 
       - name: Lint with ruff
-        run: ruff check .
+        run: python3 -m ruff check .
 
       - name: Format check with ruff
-        run: ruff format --check .
+        run: python3 -m ruff format --check .
 
       - name: Run tests
-        run: pytest -q
+        run: python3 -m pytest -q