remove black formatting in notebooks

[raphaeldussin]

fails in PR #24 for no obvious reason

@andersy005 do you want to chime in?

[aulemahal]

I found the error!
The prenotebook hook uses prettier to prettify the notebook markdown cells (and code cells if --prettier-code is given). prettier is written in javascript and a legacy version is shipped with prenotebook. However, to run, it needs nodejs, which I did not have on my machine, but is somehow accessible in github's actions.

A fix for this would be to add nodejs in ci/environment.yml.