Add PEP 517 to pyproject.toml

[pganssle]

This puts a new minimum on the PEP 518 requirement for setuptools because older versions of setuptools' PEP 517 backend will fail to include setup.py in an sdist.

I will note that the presence of pyproject.toml alone is enough to trigger PEP 517 in the latest version of pip, using the default backend, so regardless of whether or not build-backend is added to pyproject.toml, the bounds on setuptools need to be bumped (in pyproject.toml only).

[reaperhulk]

So without this patch if setuptools happens to be older than 40.6 when creating an sdist via python setup.py sdist the setup.py won't be included in the sdist?

It looks like the comment in pyproject.toml about keeping the info in setup.py and this file in sync is no longer accurate (at least for setuptools) though?

[pganssle]

So without this patch if setuptools happens to be older than 40.6 when creating an sdist via python setup.py sdist the setup.py won't be included in the sdist?

No, this is a bug with the PEP 517 build backend, so it will fail with tox or pep517.build or anything else that uses the PEP 517 build backend.

It's probably a pretty rare use case to build an sdist from source, but better safe than sorry.

[reaperhulk]

Okay, got it. Could you update the comment about keeping the setup_requirements and this in sync that notes why setuptools versioning doesn't match? Other than that this LGTM.

[pganssle]

@reaperhulk Updated, let me know if the comments need further changes.

[reaperhulk]

For reference this PR was unrelated to the bug in cryptography 2.6 (which was an issue with wheel building resolved by re-releasing as 2.6.1 after correcting our wheel builder scripts)