Matplotlib 3.3 compatibility fixups
Closes #34850 
Closes https://github.com/pandas-dev/pandas/issues/35350

Haven't tested with matplotlib's older than 3.2 yet. We'll see what CI says.
cc @jklymak. The only change I'm uncertain about being a bug vs. deliberate is https://github.com/pandas-dev/pandas/pull/35393#discussion_r459519522. > do we need to bump mpl min? (I am not sure what the oldest we test)

Matplotlib has supported datetime64 since v2.2.0, though there have been some bug fixed along the way.... 2.2.0 is our minimum right now, so I think we're good. thanks @TomAugspurger 