Replace the ipykernel dependency with an IPython dependency?

[martinRenou]

Describe the issue

In the setup.py file and in the conda-forge recipe, there is an ipykernel dependency that we could maybe get rid of. We should probably reduce the dependency to an IPython dependency, which we use directly in the codebase.

[martinRenou]

The conda-forge bot is complaining about the following:

Missing dependencies:
- traitlets
- jupyter-packaging
- numpy
- ipython
- pillow
- ipython_genutils

Unused dependencies:
- six
- ipykernel

[stonebig]

is it still mandatory to have the contraint ipython<8 ? (ipympl is the only package with that in my stack)
and recompiling it is a not trivial problem