> At first glance, Nadeem’s proposed fix is not right: paths in MANIFEST.in use '/', but then filelist produces paths using os.sep, so that the MANIFEST file and other operations done by the sdist command use native paths.  So even though the currently supported OSes all accept '/', I think the right thing is to use os.sep.

Yes, that sounds like a better solution. So the solution then is to fix
the regexps to use os.sep instead of "/" (and ensure that FileList uses
native paths throughout).

As an aside, it seems that the failing test from issue 13193 was
actually correct, and that the library itself was broken. I suppose all
of the tests will need to be changed to use native paths when this issue
is fixed.