Add warning for potential extras_require misconfiguration

[frenzymadness]

Fixes: #3467

Summary of changes

See the linked issue, all details are there.

Closes #3467

Pull Request Checklist

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

[frenzymadness]

Some of the jobs in CI are already green so this shouldn't break setuptools. I'm gonna add one more test.

[abravalheri]

Thank you very much @frenzymadness for the work in this improvement. I added a few review comments/suggestions (but please feel free to disagree or propose alternatives).

[frenzymadness]

I believe I addressed all the comments.

[frenzymadness]

I have one fixup commit there. Don't forget to squash it to the previous one before merging. Thanks for helping me with this!

[abravalheri]

I was writing the following message before I noticed that I did a mistake in the refactoring and went back to correct it:

Hi @frenzymadness, thank you very much for the modifications!
I did some refactoring on the file to better integrate with your changes (I also refactored a little bit your code). I hope you don't mind. (The idea was to speed up the ping pong review process, please feel free to change it, if you think it is not appropriate).

But I guess you already noticed that 🤣

I have one fixup commit there. Don't forget to squash it to the previous one before merging. Thanks for helping me with this!

Ok, I will do it, thank you for the pointer.

[abravalheri]

Thank you very much @frenzymadness, I plan to merge this PR together with a few other ones in a new release soon.