POC: Creation of a Comm Python package

[martinRenou]

Related discussions:

• Generic way for kernels to expose comms #342
• Remove ipykernel and IPython as dependencies jupyter-widgets/ipywidgets#3514

Currently working on a Python comm package on https://github.com/martinRenou/comm for now, but it should probably be moved in the IPython org

This has the benefit of being able to:

• remove the ipykernel dependency in ipywidgets Using the new Comm Python package jupyter-widgets/ipywidgets#3533
• remove the ipykernel monkey patch in xeus-python POC: Creation of a Comm Python package #548
• remove the ipykernel package mock in jupyterlite's pyolite
• remove the ipykernel package mock in emscripten-forge