Fix editable --user installs with build isolation

[hexagonrecursion]

Summary of changes

Make setup.py develop --user install to the user site packages directory even if it is disabled in the current interpreter.

Closes #3019

Pull Request Checklist

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

[abravalheri]

Thank you very much for investigating this @hexagonrecursion and providing a fix!

I changed a little bit your test code to avoid using directly the py library (according to their warning on https://pypi.org/project/py/, this library should not be used in new code). I hope you don't mind the changes (I just found it easy to do it directly instead of going back and forth requesting changes in the review).

[abravalheri]

This approach seems reasonable to me, if the user is explicitly asking for installing in the home, we should honour that.

Question: if I understood it correctly we are still keeping the isolation in terms of not allowing libraries installed in the user home to be used during the build, right? (The only thing that changes is that we "break the isolation" just when it is time to install the .pth file)

[hexagonrecursion]

if I understood it correctly we are still keeping the isolation in terms of not allowing libraries installed in the user home to be used during the build, right? (The only thing that changes is that we "break the isolation" just when it is time to install the .pth file)

correct

[abravalheri]

Thank you very much @hexagonrecursion, this looks good!

[hexagonrecursion]

@jaraco ping

[jaraco]

LGTM

[kynan]

FTR: the first setuptools release to include this fix is 62.0.0 from 2022-04-04