Update CI to use Python 3.13

Author: asmeurer

.github/workflows/tests.yml

@@ -5,7 +5,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ['3.9']
+        python-version: ['3.13']
       fail-fast: false
     steps:
       - uses: actions/checkout@v2