Upgrade all Tox dependencies before running Tox.

.github/workflows/tox.yaml

@@ -18,7 +18,7 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install codecov coveralls flake8 mypy pylint tox vulture
+          pip install -U codecov coveralls flake8 mypy pylint tox vulture
       - name: Run Tox
         run: |
           tox