ci: run PyQt5 tests on Ubuntu 24.04 to keep testing Python 3.9

Python 3.9 is no longer available on the Ubuntu 26.04 image used by
ubuntu-latest, so the 3.9 job failed while setting up Python. The 2.11.x
maintenance line still supports Python 3.9.

Assisted-by: Claude Opus 5.5

Author: PierreRaybaut

.github/workflows/test_pyqt5.yml

@@ -18,7 +18,8 @@ jobs:
     env:
       DISPLAY: ':99.0'
 
-    runs-on: ubuntu-latest
+    # Python 3.9 is not available on newer Ubuntu runners (ubuntu-latest)
+    runs-on: ubuntu-24.04
     strategy:
       fail-fast: false
       matrix: