Add setuptools to Nutils-related requirements.txt

[MakisH]

@fsimonis tells me in #599 (comment) that we now need to specify setuptools as a requirement whenever we also specify nutils for newer PIP versions.

I have not verified this, but here is a PR to at least not forget (but it should not harm anyway).

Checklist:

• I added a summary of any user-facing changes (compared to the last release) in the changelog-entries/<PRnumber>.md. -> N/A
• I will remember to squash-and-merge, providing a useful summary of the changes of this PR.

[fsimonis]

I think this may be a side effect of the distutils deprecation in python 3.12.