[Distutils] Metadata 2.0 and single-component version numbers

[Paul Moore]

Metadata 2.0 doesn't support versions that are a single number. This
has recently been flagged as a pip issue
(https://github.com/pypa/pip/pull/1445) because pip won't handle
wheels with a single-component version.

We can obviously accept the patch for pip as a backward-compatibility
measure, but I'm not sure if that's a good idea. wheels, and much of
the new pip functionality, is based around Metadata 2.0 and the new
packaging PEPs, at least in principle, so I'd rather get a better view
on why Metadata 2.0 doesn't accept these versions. Based on that, we
can decide what to do in pip.

As a data point, pywin32 uses a single-digit version and can't be
built with the free MS compilers, so it would be an ideal candidate
for distribution as a wheel. But I've seen nothing from the project
indicating that they are even aware of wheels yet, and there may be
good reasons why wheels won't work for them (registration of WIndows
COM components, for example)

So - should Metadata 2.0 allow single-number versions? Should we be
advocating a version format change to projects like pywin32? If
neither, what will be the impact (and do people have views on whether
it's a serious enough issue to warrant pip special-casing this)?

Thanks,
Paul