ci: matrix test Python 3.7 and 3.12

We still support Python 3.6, and probably older ones too, but it's no
longer available in the ubuntu-latest image on GitHub Actions.

So instead of 3.6 and 3.9, let's matrix test the oldest and newest
Python versions we can at the moment, i.e., 3.7 and 3.12.

.github/workflows/test-suite.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       max-parallel: 5
       matrix:
-        python: [3.6, 3.9]
+        python: ["3.7", "3.12"]
 
     steps:
     - uses: actions/checkout@v4