Fix compatibility for plot_state_hinton with mpl 3.3.0 (#4739)
Fix compatibility for plot_state_hinton with mpl 3.3.0 (#4739)

In the recently released matplotlib 3.3.0 release matplotlib started
enforcing that the number of ticklabels matches the number of tick
locations. [1] This ends up breaking on the plot_state_hinton()
visualization function because the hinton plot was not setting a label
for the implicit tick at the top of the graph, even though there is no
value for it. This commit fixes this by just setting a blank label for
the unused tick so that the visualization will still work on matplotlib
3.3.0.

[1] https://matplotlib.org/3.3.0/api/api_changes.html#axis-set-ticklabels-must-match-fixedlocator-locs
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