Test on free-threaded builds of Python (#12798)

.github/workflows/main.yml

@@ -102,6 +102,43 @@ jobs:
       env:
         PYTHONWARNINGS: "error"  # treat all warnings as errors
 
+  deadsnakes-free-threraded:
+    runs-on: ubuntu-latest
+    name: Python ${{ matrix.python }} (Docutils ${{ matrix.docutils }}; free-threaded)
+    strategy:
+      fail-fast: false
+      matrix:
+        python:
+        - "3.13-dev"
+        docutils:
+        - "0.20"
+        - "0.21"
+
+    steps:
+    - uses: actions/checkout@v4
+    - name: Set up Python ${{ matrix.python }} (deadsnakes)
+      uses: deadsnakes/action@v3.1.0
+      with:
+        python-version: ${{ matrix.python }}
+        nogil: true
+    - name: Check Python version
+      run: python --version --version
+    - name: Install graphviz
+      run: sudo apt-get install graphviz
+    - name: Install dependencies
+      run: |
+        python -m pip install --upgrade pip
+        python -m pip install .[test]
+    - name: Install Docutils ${{ matrix.docutils }}
+      run: python -m pip install --upgrade "docutils~=${{ matrix.docutils }}.0"
+    # markupsafe._speedups has not declared that it can run safely without the GIL
+    - name: Remove markupsafe._speedups
+      run: rm -rf "$(python -c 'from markupsafe._speedups import __file__ as f; print(f)')"
+    - name: Test with pytest
+      run: python -m pytest -vv --durations 25
+      env:
+        PYTHONWARNINGS: "error"  # treat all warnings as errors
+
   windows:
     runs-on: windows-2019
     name: Windows