[Distutils] PEP 566 - Package metadata version 2.1

[Thomas Kluyver]

I'd like to once again prod this PEP towards completion:

https://www.python.org/dev/peps/pep-0566/

The version numbering question has been decided in favour of calling it 2.1.

The remaining question I'm aware of is whether to make the body text (in the email format of the metadata file) officially represent the package long description. I'm in favour of doing so: at least twine and flit already use this for metadata in wheels.

Thomas