Trigger deprecation warning when find_packages() finds reserved name

[mgorny]

Summary of changes

Trigger SetuptoolsDeprecationWarning whenever find_packages() finds a top-level package whose name is found in reserved package name list, e.g. "tests". Installing these packages is probably always wrong, yet it is still a frequent mistake in packages using setuptools. This warning should increase the chance of the mistake being caught before releasing.

Closes #3243

Pull Request Checklist

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

[mgorny]

I have to admit that I went with the "minimal change" approach. If you prefer it done some other way, please let me know.

[mgorny]

I'm sorry but due to health issues I'll have to limit my computer activity for some time (apparently up to 4 weeks), so I won't be able to look into it any time soon.

[abravalheri]

Sorry to hear that... I wish you a good recovery.

[mgorny]

Thank you very much.

[jaraco]

@mgorny I hope you're back to health. I thought I'd check in on this PR and see if you wanted to continue with it?