Use os.path.commonpath() instead of commonprefix() - #222

Thanks @sethmlarson!

Would be nice to see a release of this. Because it's unreleased and it's breaking Setuptools' test suite, we'll have to suppress the warning until a release is cut (and then remove the suppression later).

(to be fair, Setuptools' own warnings-as-errors setting is also implicated)

Closes #221 and #223

FWIW, we found this switch created a breaking regression on pip: python/cpython#149444 / pypa/pip#14001

And the solution in the pip PR has a regression, that no one has reported yet, where / incorrectly can't be selected.