And another try...

apparantly githib actions do not like double quotes - although they are
part of YAML standard.

.github/workflows/tests.yml

@@ -21,18 +21,18 @@ jobs:
     strategy:
       matrix:
         os:
-          - ["ubuntu", "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:
-          python-version: "3.9"
+          python-version: '3.9'
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
           pip install tox
       - name: Test with tox
         run: |
-          tox run -e "lint,py39"
+          tox run -e 'lint,py39'