Update coverage.yml to use optuna@maser.

.github/workflows/coverage.yml

@@ -49,11 +49,14 @@ jobs:
         pip install --progress-bar off .[test]
         pip install --progress-bar off .[all]
 
+        # Install optuna from optuna master
+        pip install git+https://github.com/optuna/optuna@master
+
         # Install distributed to run importance_tests/test_init tests.
         pip install distributed
 
         echo 'import coverage; coverage.process_startup()' > sitecustomize.py
-    
+
     - name: Output installed packages
       run: |
         pip freeze --all