Rewrite workflows using native Gitea Actions shell commands instead of marketplace actions

.gitea/workflows/ci.yml

@@ -10,16 +10,20 @@ jobs:
   test:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - name: Checkout code
+        run: |
+          mkdir -p ${{ github.repository }}
+          git clone https://gitea.com/${{ github.repository }} .
+          git checkout ${{ github.sha }}
       - name: Set up Python
-        uses: actions/setup-python@v5
-        with:
-          python-version: '3.11'
+        run: |
+          apt-get update
+          apt-get install -y python3 python3-pip
+          python3 -m pip install --upgrade pip
       - name: Install dependencies
         run: |
-          python -m pip install --upgrade pip
-          pip install -e .
-          pip install pytest
+          pip3 install -e .
+          pip3 install pytest
       - name: Run tests
         run: |
-          python -m pytest tests/ -v
+          python3 -m pytest tests/ -v