Use older tox version.

See tox-dev/tox#2638 for details.

.github/workflows/tests.yml

@@ -66,7 +66,7 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install tox
+        pip install "tox < 4"
     - name: Test
       run: tox -e ${{ matrix.config[1] }}
     - name: Coverage