matplotlib.pyplot is missing from v2.1 intersphinx inventory #6695

mpl 2.1 is out, and due to their doc restructuring, the module link for matplotlib.pyplot is missing from the new intersphinx inventory. While this issue is best addressed upstream, we my also want to fix our doc warnings in the meantime, either by removing linking those 3 places, or hard-wire a link, or using the inventory for the previous version (this latter is the most hacky way, as normally astropy-helpers provides the links that we would need to overwrite now).

The upstream issue is here: matplotlib/matplotlib#9331