Run Python 3.9 CI jobs on ubuntu-24.04

ubuntu-latest now points to Ubuntu 26.04, for which actions/setup-python
provides no Python 3.9 build, so every 3.9 Linux job failed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: Pierre-Sassoulas

.github/workflows/test.yml

@@ -33,13 +33,25 @@ jobs:
         qt-lib: [pyqt5, pyqt6, pyside6]
         os: [ubuntu-latest, windows-latest, macos-latest]
         include:
+          # Python 3.9 is not available on ubuntu-latest (Ubuntu 26.04).
+          - python-version: "3.9"
+            qt-lib: pyqt5
+            os: ubuntu-24.04
+          - python-version: "3.9"
+            qt-lib: pyqt6
+            os: ubuntu-24.04
+          - python-version: "3.9"
+            qt-lib: pyside6
+            os: ubuntu-24.04
           - python-version: "3.9"
             qt-lib: pyqt61
-            os: ubuntu-latest
+            os: ubuntu-24.04
           - python-version: "3.9"
             qt-lib: pyside60
-            os: ubuntu-latest
+            os: ubuntu-24.04
         exclude:
+          - python-version: "3.9"
+            os: ubuntu-latest
           # No PySide6 release supports Python 3.15 yet (all declare
           # Requires-Python <3.15), so it cannot be installed.
           - python-version: "3.15"
@@ -64,7 +76,7 @@ jobs:
       run: |
         python -m pip install --upgrade pip
         pip install tox
-        if [ "${{ matrix.os }}" == "ubuntu-latest" ]; then
+        if [ "${{ runner.os }}" == "Linux" ]; then
           sudo apt-get update -y
           sudo apt-get install -y libgles2-mesa-dev
         fi