include pep660 proof of concept

[dholth]

Summary of changes

The beginning of copying the old dholth/setuptools_pep660 implementation into the tree.

Run python setup.py egg_info then python setup.py editable_wheel -d /tmp/ to see the output editable wheel.

Pull Request Checklist

• Changes have tests
• News fragment added in changelog.d/.
  (See documentation for details)

[jaraco]

Thanks Daniel for doing this, especially at my request. I believe @abravalheri is tackling this issue in other PRs. @abravalheri is there anything more you want to see before closing this PR?

[abravalheri]

Hi Jason and Daniel, there is nothing else from my site.

I have been doing a series PRs targeting the feature/pep660 long-lived branch.

Indeed, I started with the be3778e commit and worked all the way up to comply with some of the requirements that were brought up in #2872 (e.g. supporting the strict mode). So probably this PR will (at least in theory) also be automatically closed (with "merged" status) once we merge feature/pep660 into main.