``hatch`` parameter for stackplot

`~.Axes.stackplot`accepts a*hatch*keyword that can take as input either a stringdescribingahatchingstyle, or (new in this version), a list of such strings, that will be applied sequentially to the layers in the stack:

The`~.Axes.stackplot` *hatch*parameter now accepts a list of stringsdescribing hatchingstyles that will be applied sequentially to the layers in the stack:

..plot::

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