[DO NOT MERGE] Fixes #3548 - add a pyproject.toml

[alex]

Don't merge until PIP X is out.

[alex]

pip x comes out in a week. I'd like to make a radical proposal:

1. We drop setup_requires entirely from our setup.py; with pip x you'll get cffi installed, if you're on an older pip or setuptools you can either upgrade to pip x or manually install cffi
2. We make setup.py error cleanly if you don't have cffi installed, telling you about (1)
3. We delete all the wild code for disabling setup_requires if the setup.py command doesn't need it.

Reasons:

1. Encourage people to upgrade to pip x
2. Working around this is easy, just pip install cffi>1.7 if you can't upgrade pip for any reason
3. Let's us delete tons of obscure code
4. Helps kill off easy_install once and for all, thus protecting against scenarios like "using macOS<10.13 with system python and pip install cryptography successfully downloads cryptography, but then installing it fails to download cffi since that download is done by setuptools"

[reaperhulk]

Top 10 versions (including both pip and setuptools) used to install cryptography 2.2.2 (latest version) since it was released:

I'm baffled why 1.5.4 is so enormous. It isn't on any other PyCA project.

[dstufft]

https://packages.ubuntu.com/trusty/python-pip

[alex]

This is blocked on:

• TypeError on Python 2.7.3, as seen with pip 10 on debian wheezy msgpack/msgpack-python#299
• Running get-pip in an environment with no pip installed gives "requirement already satisfied" pypa/get-pip#19

[alex]

Turns out pip x only supports pyproject.toml with packages distributed as wheels, so we can't use it yet:

pypa/pip#5229

[alex]

Closing in favor of #4363