text: Title: Several test failures due to matplotlib no longer auto-flattening inputs to pcolormesh
  Body: Context: We noticed some seaborn failures downstream when testing on the nightly matplotlib wheels.
  
  It turns out that a recent change in matplotlib's dev branch (https://github.com/matplotlib/matplotlib/pull/24638) is causing `matrix._HeatMapper._annotate_heatmap` to fail because calls to pcolormesh are no longer flattened and by consequence changes the return value of `get_facecolor`.
  
  There are also various test failures in `test_matrix` and `test_distribution` which fail due to comparisions between flattened & non-flatttened arrays.