Wrap 'subdir/line' in PosixPath to ensure the output uses posix path separators. Ref python/cpython#103661.

Author: jaraco

importlib_metadata/__init__.py

@@ -538,7 +538,13 @@ def _read_files_egginfo_installed(self):
             return
 
         ret = [
-            str((subdir / line).resolve().relative_to(self.locate_file('').resolve()))
+            str(
+                pathlib.PurePosixPath(
+                    (subdir / line)
+                    .resolve()
+                    .relative_to(self.locate_file('').resolve())
+                )
+            )
             for line in text.splitlines()
         ]
         return map('"{}"'.format, ret)