Need to specify a version of Python version after all

Otherwise, actions/setup-python would be a no-op, falling back on the
system version of Python - if it exists.

Author: DimitriPapadopoulos

.github/workflows/tests.yml

@@ -81,6 +81,8 @@ jobs:
     steps:
       - name: Setup Python
         uses: actions/setup-python@v6
+        with:
+          python-version: 3.x
       - uses: actions/checkout@v6
         with:
           persist-credentials: false