CI: Update newest python to 3.13

Author: DWesl

.github/workflows/ci.yml

@@ -61,7 +61,7 @@ jobs:
       matrix:
         python:
         - "3.9"  # oldest Python supported by PSF
-        - "3.12"  # newest Python that is stable
+        - "3.13"  # newest Python that is stable
         platform:
         - ubuntu-latest
         - macos-latest