In deprecated setup.py install, fall back to distutils.

[jaraco]

Instead of failing when easy_install isn't present. Closes #3143

Summary of changes

Closes #3143

Pull Request Checklist

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

[jaraco]

@hroncok Would you give this a try as a fix for #3143?

[hroncok]

Thank you! The fix itself seems to work.

However, I have some remarks about the non-fix parts of this PR.

[stefanor]

That is not a lot of time. I maintain a build-system depending on this, and I know there are a lot of others. I don't think 6 months is enough time to change them.

The system I care about is Debian's Python Package toolchain (dh-python). We implemented pypa/installer in it in late 2021, and many packages have been migrated across (but almost certainly only the easy ones). It's been on my todo list to look into pushing the rest over, but there hadn't been any urgency. I thought the setup.py install interface was too entrenched everywhere to be deprecated at short notice like this.

In practice, with our current release freeze, I can't even look at starting making large-scale changes to do anything about this, in the next 6 months.