Make Pillow the only supported imaging library.

With the Conda package fixed, I think we can retire the old PIL support. Plus, matplotlib has an optional dependency on Pillow (not PIL), so why not make things simpler...

Also, fix a leaked file handle (only relevant on Python 3, where it causes a ResourceWarning).
It looks like it might be [available as of 2.5.0](https://github.com/python-pillow/Pillow/commit/196a48b4fd5cd388d5412d7aa0115e99b6e8c792)