Raw mode for xeus-python

[martinRenou]

As pointed out by @maartenbreddels, xeus-python was much faster than ipykernel to start https://github.com/voila-dashboards/voila/issues/417.

But now that xeus-python depends on IPython and imports it when starting, xeus-python might suffer from poor starting performances.

@SylvainCorlay suggested the idea of making a "raw" mode where we would not import IPython and have only the bare minimum implemented (code execution and display logic).

If the Notebook makes use of IPython magics, we might want to make a lazy import of IPython.