Test Python 3.13 on CI

As this is being integrated, Python 3.13 is no longer a release
candidate, but this marks it experimental since that might also be
used temporarily to prevent some of the failures mentioned in #1955
from failing the workflow.

But right now this allows the workflow to fail.

Author: EliahKagan

Diff for: .github/workflows/pythonpackage.yml

@@ -25,6 +25,8 @@ jobs:
           python-version: "3.7"
           os-ver: "22.04"
         - experimental: false
+        - python-version: "3.13"
+          experimental: true
 
       fail-fast: false