Attempted to fix experimental build for Python 3.13 in GitHub Actions;

.github/workflows/tests.yml

@@ -46,7 +46,7 @@ jobs:
       - name: Set up Python 3.13
         uses: actions/setup-python@v4
         with:
-          python-version: 3.13
+          python-version: 3.13.0-alpha.4
       - name: Install dependencies
         run:
           python -m pip install --upgrade pip