Define .[optional] and use it in workflows

.github/workflows/python-coverage.yml

@@ -22,6 +22,7 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --progress-bar off --upgrade pip setuptools
+        pip install --progress-bar off .[optional]
         pip install --progress-bar off .[test]
         pip install --progress-bar off "optuna>=3.0.0"
         pip install --progress-bar off .