Make Pillow the only supported imaging library.

Matplotlib already optionally depends on Pillow (not PIL!), so it's nice
to keep things simple. Plus, there are a few minor differences between
them that will just make supporting both troublesome.
Fix ResourceWarning of Image data.

If a Pillow Image is opened and only the metadata is used, then the
backing file handle is not closed, and must be done explicitly.
Close some figures when tests complete.

Unclosed figures cause warnings on subsequent tests.
Merge pull request #494 from QuLogic/close-test-figures

Close some figures when tests complete.
Merge pull request #493 from QuLogic/pillow

Make Pillow the only supported imaging library.