Update the JupyterLite config

[jtpio]

Recently JupyterLite introduced a jupyterlite-core package to avoid bringing the Pyodide and JavaScript kernels by default:

• https://github.com/jupyterlite/jupyterlite/pull/994
• https://github.com/jupyterlite/jupyterlite/pull/1013

This should help simplify the config uses in this repo.

Also the Xeus Python kernel now supports defining the packages in an environment.yml file.

More info in https://jupyterlite.readthedocs.io/en/latest/changelog.html

[jtpio]

Looking fine on the ReadTheDocs preview:

[maartenbreddels]

awesome, thank @jtpio ! The failures are not related and are due to the new ipywidgets release.