Fix python lint errors

optuna_dashboard/_app.py

@@ -257,7 +257,7 @@ def create_app(storage: BaseStorage) -> Bottle:
     @app.get("/static/<filename:path>")
     def send_static(filename: str) -> BottleViewReturn:
         if "gzip" in request.headers["Accept-Encoding"]:
-            gz_filename = filename.strip('/\\') + ".gz"
+            gz_filename = filename.strip("/\\") + ".gz"
             if os.path.exists(os.path.join(STATIC_DIR, gz_filename)):
                 filename = gz_filename
         return static_file(filename, root=STATIC_DIR)