Please don't drop setup.py install

[jaraco]

[The six-month deadline added in #4970] 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.

Originally posted by @stefanor in #4970 (comment)

[jaraco]

I see this is a duplicate of #2088, let's revisit there.