Add pyproject.toml file?

[jamadden]

pip 10 supports pyproject.toml files as per PEP518. In theory this lets us declare our build-time dependencies (notably on Cython) so that a source install can simply be 'pip install gevent.tar.gz' without having to manually install Cython:

[build-system]
requires = ["setuptools", "wheel", "Cython >= 0.28.2"]

There's a problem with this, however.

Unfortunately, pip 10 will only install wheels for things listed in requires. If you're on a platform without a binary wheel for Cython (e.g., FreeBSD or Python 3.7b3 on any system), then the installation simply bails:

 Could not find a version that satisfies the requirement Cython>=0.28.2 (from versions: )
No matching distribution found for Cython>=0.28.2

Because pyproject.toml also specifies build isolation, installing Cython ahead of time also fails, unless you specify the --no-build-isolation flag. So until pip supports installing build deps from source, it's not clear that pyproject.toml is a net win.

[jamadden]

With the resolution of pypa/pip#5229 in pip 18, source instals of dependencies are now supported. But it may be awhile before we can count on source installers to have pip 18?