matplotlib v3.3.0 deprecations

[prisae]

Matplotlib v3.3.0 introduced many new features and deprecations, which will fail two minor versions later, see

• https://matplotlib.org/3.3.0/users/whats_new.html
• https://matplotlib.org/3.3.0/api/prev_api_changes/api_changes_3.3.0/deprecations.html

Several of these affect discretize (mainly View.py), and we have to take care of them sooner or later (currently they just generate a lot of warnings for users on matplotlib v3.3.0, but no errors). Amongst others:

• pcolormesh and pcolor have a new parameter shading (https://matplotlib.org/3.3.0/users/whats_new.html#pcolor-and-pcolormesh-now-accept-shading-nearest-and-auto).
• Parameters norm and vmin/vmax should not be used simultaneously (https://matplotlib.org/3.3.0/api/prev_api_changes/api_changes_3.3.0/deprecations.html#parameters-norm-and-vmin-vmax-should-not-be-used-simultaneously)

This list is likely to grow, I seem to stumble upon new warnings and errors frequently since I updated. I think the following does not affect discretize though:

• For symlog-scales, the keywords linthreshx and linthreshy have both be renamed to linthresh.
• set_xticks(x) and set_xticklabels(labels): if x and labels have different dimensions it was ignored silently until now, but now it raises a ValueError.

[prisae]

Related to #215 and #217

[prisae]

I think you fixed these all (in #227 mostly, maybe in others too), right @jcapriot? I am closing this, please re-open if I am wrong.