ensure relpath uses /

pex/third_party/__init__.py

@@ -123,7 +123,7 @@ def _filter_names(self, relpath, pattern, group):
     # We use '/' here instead of os.sep because the zip file format spec specifies that paths must
     # use forward slashes. See section 4.4.17 of
     # https://pkware.cachefly.net/webdocs/casestudies/APPNOTE.TXT
-    relpath_pat = '{}/'.format(relpath) if relpath else ''
+    relpath_pat = '{}/'.format(relpath.replace(os.sep, '/')) if relpath else ''
     pat = re.compile(r'^{}{}{}$'.format(self.prefix, relpath_pat, pattern))
     with contextlib.closing(zipfile.ZipFile(self.zipfile_path)) as zf:
       for name in zf.namelist():