CI: Use vanilla Python 3.12 (no -dev)

Author: amotl

.github/workflows/tests.yml

@@ -22,7 +22,7 @@ jobs:
     strategy:
       matrix:
         os: ["ubuntu-latest"]
-        python-version: ["3.7", "3.8", "3.9", "3.10", "3.11", "3.12-dev"]
+        python-version: ["3.7", "3.8", "3.9", "3.10", "3.11", "3.12"]
       fail-fast: false
 
     env: