Update to jupyterlite-xeus #17

Currently repo2jupyterlite depends on 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