Update to `jupyterlite-xeus`

[jtpio]

Currently repo2jupyterlite depends on jupyterlite-xeus-python:

https://github.com/jupyterlite/repo2jupyterlite/blob/5c848e5f21c60297ceff0e2558b9abea3dc705b8/setup.py#L17

However there has been some refactoring lately to make it easier to load xeus kernels more easily in JupyterLite, with the creation of a new loader package: https://github.com/jupyterlite/xeus

Updating to this new package should hopefully be straightforward. For reference there is a migration guide: https://xeus-python-kernel.readthedocs.io/en/latest/migration.html

[jtpio]

cc @martinRenou @DerThorsten for reference, as another place where this package will be used