t platform-specific distributions must be compatible with.  If
unspecified, it defaults to the current platform.  `python` is an
optional string naming the desired version of Python (e.g. ``'3.6'``);
it defaults to the current version.

You may explicitly set `platform` (and/or `python`) to ``None`` if you
wish to map *all* distributions, not just those compatible with the
running platform or Python version.
N)