Avoid docs workflow on Python 3.10..3.12 due to distutils errors

.github/workflows/main.yml

@@ -118,6 +118,10 @@ jobs:
       - uses: actions/checkout@v3
       - name: Setup Python
         uses: actions/setup-python@v4
+        with:
+          python-version: "3.9"
+          # Cannot run on 3.12, see sphinx-doc/sphinx#11720.
+          # Also we have to avoid the deprecation error introduced in 3.10.
       - name: Install tox
         run: |
           python -m pip install tox