On Linux, converting IsADirectoryError to ZoneInfoNotFoundError is easy.

But on Windows, I don't think that converting any PermissionError into a ZoneInfoNotFoundError is a good idea. Maybe if PermissionError happens, we should check if the path is a directory.

Pseudo-code:

try:
  <open and read file>
except OSError as exc:
  if os.path.isdir(zone_path):
    raise ZoneInfoNotFoundError
  else:
    raise