[doc] Recommend not integrating with setuptools

[jamesmyatt]

As discussed in #5534 (comment), I thought it was worth including a notice that integration with setuptools should not be used. I still see projects using setup.py test and the pytest docs seems like the most discoverable place for people to find this.

[RonnyPfannschmidt]

thanks :)

[bluetech]

The text looks good to me, thanks for adding it.

But if someone only glances the best-practices document and sees this title, they will get exactly the wrong idea. Can we make the title itself clear that this is a bad practice?

[jamesmyatt]

@bluetech , I re-used the old title (see https://github.com/pytest-dev/pytest/pull/5546/files) so that existing links to the page would go to the right place. But I can change it if you prefer.

[bluetech]

Yes, I think we should change it. In this case, it's not a bad thing to break the URL :) (Also, the PR you linked already broke the link 2 years ago)

[jamesmyatt]

@bluetech, Done

[bluetech]

Thanks!