CI: Test out nbqa for linting notebooks

[MridulS]

We can use https://github.com/nbQA-dev/nbQA to lint and format our notebooks, it can run many linters like black, isort, pyupgrade and also format the markdown cells in the notebook.

#45

[MridulS]

@rossbar can you have a look here? This PR adds a linting step to our CI with pre-commit and nbQA.

[rossbar]

Thanks for this @MridulS , I like this idea and the setup is pretty straightforward. It would be super awesome if the tooling around notebooks (nbQA and nbval come to mind) supported the text-based markdown format, then we could pull this off without all the jupytext conversions (and actually use pre-commit in the repo rather than only in CI!).

That's beyond the scope here though, this is definitely an improvement IMO!

One thought re: organization: would it be worth cleaning up the history a bit and squash everything down to two commits: one where the CI infrastructure is added, and another where the linting is applied to the existing notebooks? That way, we can do the .git-blame-ignore-revs thing for the commit with the content changes. It's probably not super important for the nx-guides history but it might be nice to do if it's not too much trouble. Other than that, LGTM! I'll let you make the decision re: history munging and please feel free to self-merge or ping me again!

[rossbar]

One thought re: organization: would it be worth cleaning up the history a bit and squash everything down to two commits: one where the CI infrastructure is added, and another where the linting is applied to the existing notebooks?

Meh let's not let this be a blocker. In this goes, thanks for setting this up @MridulS !