Further fiddling around with workflows.

.github/workflows/tests.yml

@@ -21,16 +21,14 @@ jobs:
     strategy:
       matrix:
         os:
-          - ubuntu-latest
+          - ["ubuntu", "ubuntu-latest"]
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v4
       - name: Set up Python (3.9)
         uses: actions/setup-python@v5
         with:
-          # We can give multiple python versions, but no py2.x any more
-          python-version: |
-            "3.9"
+          python-version: "3.9"
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip