Problem

While addressing the matplotlib.cm.get_cmap deprecation in 3.6:

PendingDeprecationWarning: The get_cmap function will be deprecated in a future version. Use ``matplotlib.colormaps[name]`` instead.

I noticed that None isn't directly migrate-able

In [1]: import matplotlib

In [2]: matplotlib.cm.get_cmap(None)
Out[2]: <matplotlib.colors.ListedColormap at 0x11e609e20>

In [3]: matplotlib.colormaps[None]
KeyError: 'None is not a known colormap name'

Proposed solution

It appears from the source that get_cmap(None) defaults to matplotlib.rcParams['image.cmap'] so it would be nice if colormaps[None] could default to that as well.

Otherwise, it would be nice if this was better documented.