clarify legacy setup.py error message further

[graingert]

a follow up to #1467

python projects should commit to a strong consistency of message regarding packaging. eg we shouldn't tell people to add a setup.py to their already configured pep-517 project, otherwise it could imply that pyproject.toml isn't as well supported and recommended as it truly is

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[gaborbernat]

[gaborbernat]

Can you please fix the tests; and add one for pyproject.toml?

[graingert]

@gaborbernat new test added, and it should pass CI now