I think it may be good enough to add a warning on skipped files in _unpack_zipfile().

 - this way we keep backwards compatibility (especially since behavior in both modules differed for such a long time.)

 - it's not clear that ZipFile behavior is superior -- for example, what if a file with stripped path components overwrites existing files?

 - if requested in the future, a parameter can be added to enable ZipFile-like behavior

 - it can be very confusing if files are silently skipped, especially if an archive has thousands of files. 

I've added a PR, note that the test in PR also tests that files with '..' are indeed skipped, we don't have a test for that now, so that's an added benefit.