fix: get_metadata deprecated since traitlets 4.1

[ManonMarchand]

Hello,

This is a very tiny PR, but I get a deprecation warning raised by this line from the traitlets library when using a custom ipywidget:

https://github.com/ipython/traitlets/blob/cb672eb5ba235fdfee6407bd7f1b1985de934e7c/traitlets/traitlets.py#L833

Here is a fix that should remove it?

Edit: I explicitly allow anyone to edit anything I might have done wrong, I have no idea why the tests are failing

[github-actions[bot]]

:point_left: Launch a binder notebook on branch _ManonMarchand/ipywidgets/fix-getmetadata