Constrain docs workflow to Python 3.13 (#248)

Author: speller26

.github/workflows/docs.yml

@@ -14,7 +14,7 @@ jobs:
       - uses: actions/checkout@v5
       - uses: actions/setup-python@v5
         with:
-          python-version: "3.x"
+          python-version: "3.13"
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip