build: Add ipykernel optional dependency to dev group

[dangotbanned]

This PR adds the optional ipython dependency ipykernel, to avoid the following issue.

Bug

Attempting to run a notebook

Install failing due to uv instead of pip

Error command

This has to be modified to fix:

# >>> python -m pip install ipykernel -U --force-reinstall
>>> python -m uv pip install ipykernel -U --force-reinstall

---

I have been manually working around this since we moved from pip -> uv in #3431 (comment)

But this change is much simpler and doesn't need to be repeated for each new environment.

[binste]

Useful, thanks!