fixing for lint

Author: BSd3v

dash/dash.py

@@ -525,7 +525,7 @@ def __init__(  # pylint: disable=too-many-statements, too-many-branches
 
         # Determine backend
         if backend is None:
-            backend_cls = get_backend('flask')
+            backend_cls = get_backend("flask")
         elif isinstance(backend, str):
             backend_cls = get_backend(backend)
         elif isinstance(backend, type):