cffi: fix a bytes vs str issue on macOS when listing directories

This code hasn't been touched in recent years, and the other implementation
return bytes for the filename, so I assume this is a holdover from the py2 days.
I was unable to test it on mac though, because the `_osutil` import failed.

mercurial/cffi/osutil.py

@@ -57,7 +57,7 b' if pycompat.isdarwin:'
                 ofs = cur.name_info.attr_dataoffset
                 str_lgt = cur.name_info.attr_length
                 base_ofs = ffi.offsetof(b'val_attrs_t', b'name_info')
-                name = str(
+                name = bytes(
                     ffi.buffer(
                         ffi.cast(b"char*", cur) + base_ofs + ofs, str_lgt - 1
                     )