[INFRA] add (optional) pre-commit config file

[sappelhoff]

closes #928 the proposal by @Remi-Gau (with which I agree)

people can optionally use this by doing:

1. pip install pre-commit and then
2. pre-commit install (from the root of the repo)

We could potentially even run the hooks in the CI and raise an error if any fail 🤔

Like here: https://github.com/executablebooks/sphinx-copybutton/blob/6ee971ab8f9c3741d380240c180b1ff47f1d2ddc/.github/workflows/integration.yml#L22-L24

[Remi-Gau]

Are the file changed a result of running pre-commit run -a?

Maybe we should mention the use of pre-commit in the CONTRIBUTING.md?

[sappelhoff]

Are the file changed a result of running pre-commit run -a?

yes, but if we do it in CI, we don't commit those changes --> we would just see if there are any, and then raise if True.

The argument against adding pre-commit to the CI is that it might confuse contributors and take away a bit of the "optionality" of pre-commit as we want to introduce it right now

Maybe we should mention the use of pre-commit in the CONTRIBUTING.md?

will do

[Remi-Gau]

The argument against adding pre-commit to the CI is that it might confuse contributors and take away a bit of the "optionality" of pre-commit as we want to introduce it right now

yeah for the moment I would hold back from using the pre-commit CI: it is great and uber fast but it can really confuse people if they don't have pre-commit installed locally.

[Remi-Gau]

This looks good to me. I can't wait to have this merged in.

[effigies]

Looks reasonable