Bug#966393: pandas: FTBFS with matplotlib 3.3

[Rebecca N. Palmer]

Control: tags -1 - patch

That's not enough by itself: it does fix that particular error, but 
exposes 16 other failures, some of the form

ValueError: The number of FixedLocator locations (8), usually from a 
call to set_ticks, does not match the number of ticklabels (4).

and some datetime issues.