DOC: docs builds are failing

[neutrinoceros]

Bug report

Bug summary

Error message

Sphinx parallel build error:
nbsphinx.NotebookError: NoSuchKernel in cookbook/custom_colorbar_tickmarks.ipynb:
No such kernel named python3
make: *** [Makefile:51: html] Error 2
Build step 'Execute shell' marked build as failure

example logs
Seen twice in #4923

ping @Xarthisius

[yut23]

Looks like ipykernel is missing: https://nbsphinx.readthedocs.io/en/0.9.4/installation.html#Jupyter-Kernel
The previous successful build happened to pull it in through a glueviz dependency:

Collecting ipykernel!=5.0.0,!=5.1.0,>=4.0 (from glue-qt>=0.1.0->glue-vispy-viewers>=1.0.3->glueviz>=0.13.3->yt==4.4.dev0)

[neutrinoceros]

I don't get it. yt depends on ipywidgets which in turn depends on ipykernel

[yut23]

pip show ipywidgets says it doesn't: Requires: comm, ipython, jupyterlab-widgets, traitlets, widgetsnbextension

[neutrinoceros]

Oh right, it's now an optional dependency, I missed that.
Well then it should be trivial to fix. Since we need to change docs somehow to even trigger the job, do you mind doing it with your already active PR ?

[yut23]

Do you mean #4923? It was already merged.

[neutrinoceros]

Whoops. I didn't have enough coffee. Or maybe I had too much ? 🤔

[chrishavlin]

adding ipykernel as a dep in #4925 now