fix Img.world_files

testing under Python 3, the doctest hangs because of the `map` usage

(`map` returns a lazy "map object", and extending it's argument with its result
is a problem)

this was missed in the recent partial Python3 port

using a list comprehension seems nicer than forcing the map object to evaluate
with

    list(map(_convert_basename, result))
Merge pull request #448 from njwilson23/fix-world-files-doctest

fix Img.world_files