Fix unintentional deprecation warnings

[jasongrout]

We missed a traitlet setup function that is called when the class is setup.

Fixes #3648

With these changes:

$ python -c "import warnings; warnings.resetwarnings(); import ipywidgets"
/private/var/folders/l0/km7g2yss183d7nrskwl4xgbc0000gn/T/tmpenv.Moiql2qU/env/lib/python3.11/site-packages/jupyter_client/connect.py:27: DeprecationWarning: Jupyter is migrating its paths to use standard platformdirs
given by the platformdirs library.  To remove this warning and
see the appropriate new directories, set the environment variable
`JUPYTER_PLATFORM_DIRS=1` and then run `jupyter --paths`.
The use of platformdirs will be the default in `jupyter_core` v6
  from jupyter_core.paths import jupyter_data_dir

[github-actions]

👈 Launch a binder notebook on branch jasongrout/ipywidgets/deprecationwarnings