The first release of the Matplotlib 3.0 series

This is the first version of Matplotlib to only support Python 3.

Highlights of this release include:

 - GUI backend is selected at run-time based on what toolkits are
   installed.  A GUI toolkit will not be selected on a headless
   server.
 - New cyclic color map *twilight*
 - Improvements to automatic layout of titles, ticks, and GridSpec
 - Many bug fixes!