MNT: Update test image thresholds for Matplotlib 3.3
MNT: Update test image thresholds for Matplotlib 3.3

Slight change to rotation calculation results in subtle pixel-level
changes, so update the tolerances--the changes aren't big enough to
warrant new images.

dopplershift/MetPy,Unidata/MetPy,dopplershift/MetPy,Unidata/MetPy,ShawnMurd/MetPy