Use Matplotlib built-in converters for python date(time)s
In Matplotlib we have and maintain unit converters for the python build in datetime objects and `np.datetime64`, so there is no need to provide another converter. This would fix https://github.com/pandas-dev/pandas/issues/27479
Thanks @dstansby, will take a look.

It sounds like you're more knowledgeable than most of us about matplotlib.  Can you offer any insight into #26676? Hmm this clearly isn't as easy as I thought it would be, since the pandas converters offer extra functionality.