No longer allow CI to select a prerelease for 3.12

Since 3.12.0 stable is out, and available via setup-python, per:
https://github.com/actions/python-versions/blob/main/versions-manifest.json

Author: EliahKagan

Diff for: .github/workflows/pythonpackage.yml

@@ -18,8 +18,6 @@ jobs:
         python-version: ["3.7", "3.8", "3.9", "3.10", "3.11", "3.12"]
         include:
         - experimental: false
-        - python-version: "3.12"
-          experimental: true
 
     defaults:
       run: