Describe the issue

Summary

In Matplotlib 2.1, the behavior of reusing existing axes when created with the same arguments was deprecated (see#9037). The request is to finally remove this behavior.

My immediate motivation is that I have acollection of projection classes that take as a keyword argument an instance ofastropy.coordinates.SkyCoord, whichas of Astropy 4.1 are no longer hashable (see alsoastropy/astropy#9857). This unexpectedly broke my code because I had not realized that projection keyword arguments had to be hashable.

Proposed fix

Remove the AxesStack class, remove theFigure._make_key method, and update the documentation to reflect the removal of the old deprecated behavior.