fix Img.world_files

While looking at travis testing on Python 3, I was experiencing a hang on a doctest. What's strange is that I could have sworn tests would complete (and fail) under Python 3.4 before.

__Commit message__

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
```python
list(map(_convert_basename, result))
```