Run tests on Python 3.13 release candidate

* https://www.python.org/download/pre-releases
* https://github.com/actions/setup-python/blob/main/docs/advanced-usage.md#allow-pre-releases

Blocked by:
* python-greenlet/greenlet#396

Signed-off-by: Christian Clauss <cclauss@me.com>

.github/workflows/test-python.yml

@@ -25,12 +25,13 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: Set up Python 3.12
+      - name: Set up Python 3.13
         uses: actions/setup-python@v5
         with:
-          python-version: "3.12"
+          python-version: "3.13"
           cache: "pip"
           cache-dependency-path: pyproject.toml
+          allow-prereleases: true
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3