MatplotlibDeprecationWarning for imshow keywords

[tboggs]

Example:

In [4]: w = spy.imshow(img, (30, 20, 10), classes=gt)                                                                                      
/home/thomas/src/spectral/spectral/graphics/spypylab.py:904: MatplotlibDeprecationWarning: Passing parameters norm and vmin/vmax simultaneously is deprecated since 3.3 and will become an error two minor releases later. Please pass vmin/vmax directly to the norm when creating it.
  self.class_axes = plt.imshow(self.class_rgb, **kwargs)