Switch to the xeus python kernel

[jtpio]

The Xeus Python kernel for JupyterLite allows for pre-installing packages at build time: https://xeus-python-kernel.readthedocs.io/en/latest/configuration.html#pre-installed-packages

It could be useful to switch to that kernel for this JupyterLite deployment, since the folks coming from https://jupyter.org/try might wonder why they need to call piplite.install() before using other packages.

[bollwyvl]

They may also wonder why they can't install anything else...

[jtpio]

For now. This will eventually be possible.

[bollwyvl]

The reverse could also have been true a few months ago: jupyterlite/pyodide-kernel#27

Now that pyodide-lock is packaged and supported by pyodide 0.24.0, that PR can be revisited to allay some of the concerns about complexity.