Deprecation aliases in cbook

The module matplotlib.cbook.deprecation was previously deprecated in Matplotlib 3.4, along with deprecation-related API in matplotlib.cbook. Due to technical issues, matplotlib.cbook.MatplotlibDeprecationWarning and matplotlib.cbook.mplDeprecation did not raise deprecation warnings on use. Changes in Python have now made it possible to warn when these aliases are being used.

In order to avoid downstream breakage, these aliases will now warn, and their removal has been pushed from 3.6 to 3.8 to give time to notice said warnings. As replacement, please use matplotlib.MatplotlibDeprecationWarning.