Log Message:
py-matplotlib: updated to 3.1.2

v3.1.2:
- fix regresion of incorrect limits for y-axis in imshow
- fix integer indexed pandas series
- fix ScalarFormatter formatting of masked values
- webagg and nbagg no longer incorrectly fallback to agg when $DISPLAY is not set
- several fixes for Qt5 backend
- other bugfixes and documentation fixes