Find sdist when project only provides wheels

[jayvdb]

If the package doesnt provide an sdist, or the sdist doesnt contain tests, usually one can be obtained from the git repo listed in the pypi metadata, find the corresponding tag among its tags, and then grab a tarball from the git hosting provider (usually GitHub of course).

It would be lovely to have this process automated as an optional feature which fails with only a warning when a link in that chain of steps fails.

Probably label the tarball slightly differently so it can be easily seen that it was not from pypi.