[upadate] np.rank has been deprecated

[mmcky]

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from one of our readers:

In the introduction of quantecon lecture it discussed np.rank. I could not run it so I check and according to this website

https://numpy.org/devdocs/release/1.18.0-notes.html

it looks like numpy.rank has been removed.

I still can run it on Pythonista but I cannot run it on Jupyter notebook.

You might want to check if you have the same problem and correct if the rest of the lecture implement it.