Update to jupyterlite-xeus

[jtpio]

Currently repo2jupyterlite depends on jupyterlite-xeus-python:

repo2jupyterlite/setup.py

Line 17 in 5c848e5

"jupyterlite-xeus-python",

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