REL: v2.1.2

The second bug-fix release for the 2.1 series.

This release fixes a number of critical bugs:

 - fix a typo in mlab.cohere which yielded incorrect results
 - allow nonposx / nonposy to pass through loglog
 - fix color comparisons when finding handles with legend
 - fixes a recursive draw bug and a major performance regression in
   Qt5Agg backend
 - fix a re-draw bug in WxAgg
 - fix image scaling with high-bit depth integers
 - revert the busy-cursor