REL: v3.1.0

The first release of the v3.1 series

This version of Matplotlib support Python 3.6+

Highlights of this release include:

 - A helper method for scatter legends
 - Secondary axis support (ex, degrees and radians)
 - A concise date formatter
 - No longer requires a framework build of Python to use OSX backend
 - Major and minor ticks will no longer collide by default
 - A progress callback for animations
 - mplot3D is always registered
 - deprecated mlab functions have been removed
 - Many function calls have become stricter about invalid or ignored input