[Distutils] Add data-requires-python attribute to download links in simple repository API

[Thomas Kluyver]

In a discussion about how to allow pip to select a package version
compatible with the target Python version, Donald suggested adding a
data-requires-python attribute to links in the simple repository API,
which pip uses to find candidate downloads.

<a href="..." data-requires-python=">=3">...</a>

This would expose the Requires-Python metadata field already specified
in PEP 345. There is a separate PR for setuptools to allow specifying
this value.

I have opened a PR to add this to PEP 503, which defines the simple
repository API, and Donald asked that I post about it here for comments:

https://github.com/python/peps/pull/56

Thanks,
Thomas