Run the test matrix on ubuntu-24.04

ubuntu-latest is now Ubuntu 26.04, where setup-python has no Python 3.9,
so the 3.9 job failed before installing anything; the package still
declares requires-python >= 3.9.

Author: dmeoli

.github/workflows/tests.yml

@@ -8,7 +8,7 @@ on:
 
 jobs:
   test:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-24.04
     strategy:
       fail-fast: false
       matrix: