Migrate from tox to nox

[agriyakhetarpal]

It is more of an opinionated choice because both of them are quite robust tools. Some arguments in favour of nox and how to configure it are listed the Scientific Python guides: https://learn.scientific-python.org/development/guides/tasks/#task-runners

We don't need to follow them fully, but in general, I have found them to be pretty good and reasonable guidelines :)

Note that nox provides a tox-to-nox CLI as well to ease the migration, but it isn't too difficult to do so by hand either, and it shouldn't take too long.

[agriyakhetarpal]

I might have a bias because I'm a fond user of nox 😁 so I mostly agree with all of the arguments listed here, sharing for reference: astropy/astropy#16412

Also, https://hynek.me/articles/why-i-like-nox/ is a great read!