Deprecation warning from traitlets library

[ManonMarchand]

There is a deprecation warning that is raised on some instantiations of the widget in version 0.3.0 :

DeprecationWarning: Deprecated in traitlets 4.1, use the instance .metadata dictionary directly, like x.metadata[key] or x.metadata.get(key, default)
  warn("Deprecated in traitlets 4.1, " + msg, DeprecationWarning, stacklevel=2)

This PR should solve it:

https://github.com/jupyter-widgets/ipywidgets/pull/3894