update `ipywidgets` > 8.0.4

Hello everyone,

I am wondering why nbdev limits the use of `ipywidgets` to versions <= 8.0.4 (from March 2023).

In the meantime `ipywidgets` has developed to version 8.1.2 ( https://github.com/jupyter-widgets/ipywidgets/releases )
Especially in version 8.0.7 support for JupyterLab 4 was added.

Best regards,
Jens Henrik

Start by locating nbdev's dependency declaration that limits ipywidgets to version 8.0.4 or lower. Check the project's compatibility setup against ipywidgets 8.1.2 and JupyterLab 4, then run the existing test suite; done means the newer version is allowed without breaking supported environments.

Written by the indexing model from the issue text.