Update sdist specification section to refer to PEP 517

[ncoghlan]

(Filing as an issue rather than a PR to avoid conflicting with @di's PR at #386)

https://www.python.org/dev/peps/pep-0517/#source-trees defines a new sdist format for use with PEP 517 compatible build frontends.

The relevant specific subsection should be updated to mention the provisional acceptance of that format, but note that publishing tools should still provide setup.py shims for the time-being (since it will be a while before native PEP 517 is widespread in frontends)

[ncoghlan]

https://packaging.python.org/specifications/distribution-formats/#source-distribution-format was updated to reference PEP 517 back in #472, but we didn't close this issue at the time.

I found it again while starting to file an issue pointing out that as of the pip 19 release in January 2019, PEP 517 and PEP 518 are supported now, so setup.py shims are only needed for compatibility with older installer versions - at least some new installers can handle pyproject.toml directly.