Allow file: for dependencies in TOML

[akx]

Summary of changes

This is the pyproject.toml counterpart of #3253, as requested by @abravalheri in #3253 (review).

• The _validate_pyproject code was generated using akx/validate-pyproject@61b668f

Pull Request Checklist

• Changes have tests
• News fragment added in changelog.d/
• Docs added
• JSON schema changes merged to validate-pyproject(Allow dependencies/optional-dependencies to use file directives abravalheri/validate-pyproject#37)

[abravalheri]

Thank you very much for the very quick implementation @akx.

You went even further than I was expecting and also provided a patch for validate-pyproject, that is greatly appreciated!

I left some review comments. Some of them are minor, but I think the one about _ensure_previously_set is important to implement.

Would you also be OK allowing the maintainers to implement changes in the PR? There are minor things/nitpicks that I would edit before accepting the PR, but those are really not worth bothering you.

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BTW: sorry for the trouble of requiring you to change the schemas in a different project. My long term vision is to move them to the setuptools repository.

Very great that you managed to find the way of re-generating the validation code even without proper documentation for tox -e generate-validation-code 👏 ❤️

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Regarding the acceptance, let's see how Jason review the other PR for setup.cfg. When that one gets merged, I am very happy to merge the changes here.

[akx]

I left some review comments. Some of them are minor, but I think the one about _ensure_previously_set is important to implement.

Addressed, thank you!

Would you also be OK allowing the maintainers to implement changes in the PR? There are minor things/nitpicks that I would edit before accepting the PR, but those are really not worth bothering you.

Honestly I'd prefer if you could make review comments instead, I don't mind doing the work! That'd also give me a bit more insight to the stylistic/... choices for setuptools, for future contributions.

Very great that you managed to find the way of re-generating the validation code even without proper documentation for tox -e generate-validation-code

Oh, there was a tox command..? 😁 I just looked at the NOTICE in the _validate_pyproject subdirectory. Also, fastjsonschema seems very interesting – I might have some work use for it at @valohai...

[abravalheri]

Thank you very much @akx for providing the changes.
I left a few other comments regarding the previous error in the CI and the nitpicks.

Should we also include an equivalent note in the docs about this method not being intended for pinning?

[akx]

@abravalheri Review comments addressed, docs added! :)

[abravalheri]

Thank you very much @akx, it looks very good to me.

I think we can merge this once the other PR is approved.

Any chance you can have a look on the CI to see why is it failing?

[akx]

@abravalheri The CI error was just a flake8 complaint about a line length. Fixed that, as well as the requirements.txt formatting in the docs.