Add PEP-517 support

[Adrián Chaves]

It would be great if lxml would specify cython as a required build tool as part of https://www.python.org/dev/peps/pep-0517/

At the moment, to install lxml from the latest sources using tox, you need to use a tox hack (https://github.com/tox-dev/tox/issues/167#issuecomment-393421530) which will eventually stop working, and tox developers have stated that the way forward is for Python projects to specify their build tools as per PEP-517, which tox does support.

[scoder (scoder)]

The thing is, Cython isn't required, unless you build from a clean source checkout. The source distribution of a normal release does not have any Python dependencies at all, it only needs Python and a C compiler.

I guess this could be expressed with the build-sdist hook:
https://www.python.org/dev/peps/pep-0517/#get-requires-for-build-sdist

To me, it doesn't really seem worth the hassle of setting up a local build backend just for this case. Can't say if others feel differently about this.

[Adrián Chaves (gallaecio)]

I understand your position.

For background: The reason I wish to build from the latest Git commit is to be able to run tests of a project built on top of lxml against that latest Git commit, so that we can detect and prepare for incompatibilities with future lxml versions before they are released. For more information, see https://github.com/scrapy/scrapy/issues/5291