Add an estimated date for removing setuptools.dep_util

[abravalheri]

Discussed in #4128

^{Originally posted by abravalheri November 21, 2023}
Right now we have a warning that is easily to miss out:

https://github.com/pypa/setuptools/blob/v69.0.0/setuptools/dep_util.py#L11

In the past we were criticised because of the lack of visibility (#3780), so maybe we should promote this to a SetuptoolsDeprecationWarning.

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^{Originally posted by jaraco in #4128 (comment)}

Yes, I agree. Thanks for catching that. 6 months for a due date seems suitable me. Feel free to tackle it or I'll get to it eventually.