[Distutils] Provisionally accepting PEP 517's declarative build system interface

[Thomas Kluyver]

As was suggested at some point, I have added a build_sdist hook to my
PR, with the following details:

- A brief definition of the minimal requirements of an sdist.
  - I have limited the definition to gzipped tarballs. Zip files also
  work as sdists, but we're moving towards standardising on tarballs, so
  I think it's simplest to require that of PEP-517 compliant tools.
- The build_sdist hook must be defined, but may not always work (e.g. it
may depend on a VCS)
- The prepare_build_files hook is optional, and in its absence,
frontends can use build_sdist and extract the files to create a build
directory.
- Backends (like flit) where building an sdist has extra requirements
should define prepare_build_files.

https://github.com/python/peps/pull/277/files