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

.github/workflows/main.yml

@@ -7,14 +7,15 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ["3.10", "3.11", "3.12"]
+        python-version: ["3.10", "3.11", "3.12", "3.13"]
 
     steps:
       - uses: actions/checkout@v4
       - name: Set up Python ${{ matrix.python-version }}
         uses: actions/setup-python@v5
         with:
           python-version: ${{ matrix.python-version }}
+          allow-prereleases: true
       - name: Set up Memcached
         run: |
           sudo apt update