Build docs in Python 3.11

This avoid `Literal[...]` bug, see jbms/sphinx-immaterial#275

.github/workflows/docs.yaml

@@ -29,10 +29,10 @@ jobs:
           # NOTE: Fake ternary operator, see https://github.com/actions/runner/issues/409
           fetch-depth: 0  # Fetch all commits and tags, needed for intermediate versions
 
-      - name: Set up Python 3.8
+      - name: Set up Python 3.11
         uses: actions/setup-python@v4
         with:
-          python-version: 3.8
+          python-version: 3.11
 
       - name: Upgrade pip
         run: python3 -m pip install --upgrade pip