Require an older tox version.

See tox-dev/tox#2638 for details.

config/default/tests.yml.j2

@@ -108,7 +108,7 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install tox
+        pip install "tox < 4"
 {% for line in gha_additional_install %}
         %(line)s
 {% endfor %}