Improve CI workflow: use direct Python setup instead of actions/setup-python

.gitea/workflows/ci.yml

@@ -9,10 +9,13 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
-      - uses: actions/setup-python@v5
-        with:
-          python-version: '3.11'
-      - name: Install
-        run: pip install -e . && pip install pytest pytest-asyncio
+      - name: Set up Python
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y python3 python3-pip python3-venv
+          python3 --version
+          pip3 --version
+      - name: Install package
+        run: pip3 install -e . pytest pytest-asyncio
       - name: Run tests
         run: pytest tests/ -v