Development changes

Matplotlib now requires numpy>=1.15


Matplotlib now uses Pillow to save and read pngs

The builtin png encoder and decoder has been removed, and Pillow is now a dependency. Note that when reading 16-bit RGB(A) images, Pillow truncates them to 8-bit precision, whereas the old builtin decoder kept the full precision.

The deprecated wx backend (not wxagg!) now always uses wx's builtin jpeg and tiff support rather than relying on Pillow for writing these formats; this behavior is consistent with wx's png output.