[Distutils] fix setup_requires by allowing it in setup.cfg or a setup_requires.txt

[Daniel Holth]

On Thu, May 15, 2014 at 4:59 PM, Marcus Smith <qwcode at gmail.com> wrote:

> this technique would be a great topic for the PPUG
> please post if you write it up.
>
>> since setup.py needs setup_requires to be installed before it can run,
>> this feature is crap
>
> well, you can write command extensions that depend on "setup_requires"
> projects.
> what's an example you're thinking of, for one of these "abstractions"?

Glyph may have better examples. One common example is to call some
code that figures out what the version number should be. He'd like to
publish some helpers that make it easier to package twisted extensions
correctly. cffi is another dependency that's sometimes needed during
the setup() call since it instantiates Extension().

I'm of course hoping that someone uses the feature to do a
setup.py-command-line-interface-compatible distutils replacement
(rather than a distutils extension) like what Bento does with its
pip-compatible setup.py replacement.