CI/CD: use Python 3.12 by default

For CI/CD tasks not testing a specific Python version, switch to Python 3.12.
(Had been using mixture of 3.10 and 3.11. GitHub's ubuntu-22.04 runner has
Python 3.10 installed by default, but 3.12 should be in the tool cache.)

.github/workflows/release.yml

@@ -31,7 +31,7 @@ jobs:
       - name: Setup Python
         uses: actions/setup-python@v5
         with:
-          python-version: "3.10"
+          python-version: "3.12"
 
       - name: Install build requirements
         run: |