Enabled the equal_nan keyword argument for np.array_equal() when the arguments are astropy.units.Quantity instances.

[byrdie]

The equal_nan keyword argument was added in Numpy 1.19. This PR just adds it to our overload of np.array_equal.

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[mhvk]

Nice, thanks! CI is currently broken due to a tox upgrade, but this all looks good to me, so I'll approve.