sort_file: Preserve permission bits

isort 4 would overwrite the old file in place, so its permission bits
would not be modified.  But as of commit
1920a6c4 or so, isort instead writes a
new file with default permission bits and renames it into place.  So
we now need to explicitly copy the permission bits.

Fixes #1194.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>