Use flake8

setup.py

@@ -34,11 +34,11 @@ setup(
     packages=find_packages(),
     install_requires=["optuna", "bottle"],
     extras_require={
-        "lint": ["black"],
+        "lint": ["black", "flake8"],
         "release": ["wheel", "twine"],
     },
     package_data={"optuna_dashboard": ["public/*"]},
-    test_suite='tests',
+    test_suite="tests",
     entry_points={
         "console_scripts": [
             "optuna-dashboard = optuna_dashboard.cli:main",