Use precommit to normalize codebase

[jasongrout]

I've seen several Jupyter projects adopt precommit (thanks @blink1073!). For fun, I copied JupyterLab's precommit file and tried it out here.

[github-actions]

👈 Launch a binder notebook on branch jasongrout/ipywidgets/precommit

[jasongrout]

It seems that 3251bc2 introduces a circular dependency or something - we can't even import ipywidgets

[bollwyvl]

Yep, this would be good. Would be interesting to rekick this to see if anything improves the CI situation.

[bollwyvl]

As discussed on #3673: potentially consider adding to prettier config:

printWidth: 88     # to align with `black`
proseWrap: always  # to improve narrative diffs

Also:

• jupyter-black
• and some way of formatting/normalizing markdown cells of notebooks...
  • I don't yet have a good answer for this, though not for lack of trying: prettier still seems to give better results over mdformat, but is relatively lousy at many small inputs called from python
  • perhaps a prettier-plugin-ipynb would be an option, but out of scope for ipywidgets