Bug report

Bug summary

pcolor() produces a deprecation warning with default settings.

The default of rcParams['pcolor.shading'] seems to be 'flat', which is deprecated (at least sometimes). Maybe the default should be changed to 'auto' or 'nearest'?

Code for reproduction

from matplotlib import pyplot as plt
import numpy as np

x = np.arange(2.)
y = np.arange(2.)
z = np.ones([2,2])

plt.pcolor(x,y,z)

Actual outcome

MatplotlibDeprecationWarning: shading='flat' when X and Y have the same dimensions as C is deprecated since 3.3.  Either specify the corners of the quadrilaterals with X and Y, or pass shading='auto', 'nearest' or 'gouraud', or set rcParams['pcolor.shading'].  This will become an error two minor releases later.
  plt.pcolor(x,y,z)

Expected outcome

No warning

Matplotlib version

• Operating system: Linux Mint 19
• Matplotlib version: 3.3.2
• Matplotlib backend (print(matplotlib.get_backend())): Qt5Agg
• Python version: 3.7.6
• Jupyter version (if applicable): N/A
• Other libraries: None

Installed with conda from conda-forge