Implement PEP 518 support

[jayvdb]

https://www.python.org/dev/peps/pep-0518/

pyproject.toml, if it exists, should be used like setup_requires.

See also pypa/pip#3691

[tuukkamustonen]

This seems to be supported in the forthcoming pip 10.0.

What is there to support on setuptools side? python setup.py install?

What is the plan in general - easy_install seems more or less deprecated, and pip install . (or pip install -e .) has pretty much replaced python setup.py install (or python setup.py develop). Pip seems to be the blessed tool these days. Is there any benefit in supporting two different ways (setup.py install vs pip install) for the same thing?

(pyproject.toml seems to be slowly crawling in and (while very limited at the moment) might even replace setup.py altogether?)

[pganssle]

@jaraco @benoit-pierre Is the plan here to have all PEP 518 go through pip? If so, I think we can close this as it's fixed in pip.

For the most part, PEP 518 support is difficult or impossible to achieve using the current approach, but I have created #1515 proposing a possible way forward that doesn't completely drop support for custom setup commands.

[jaraco]

I believe the vision (and my perspective) is that setuptools aims to be a target for pep 517/518 and other libraries (pip, pep517) will implement the designs of the PEPs. So my instinct is indeed "won't fix".

[pganssle]

Agreed, let's close and revisit later if things change.