"per square arcsecond" units render wrong in latex/notebook

This bug appears most clearly in the following clip from a Jupyter notebook:

And the underlying cause is apparent from looking at what the latex being rendered is:

>>> (u.arcsec**-1)._repr_latex_()
'$\\mathrm{\\frac{1}{{}^{\\prime\\prime}}}$'
>>> (u.arcsec**-2)._repr_latex_()
'$\\mathrm{\\frac{1}{{}^{\\prime\\prime}^{2}}}$'

The problem is that arcseconds are rendered as nothing to a power, so when you put that to a power, things go wrong.

The question is: what is the right answer? should it render as the arcsecond symbol with a 2 next to it? Or do we think it’s wrong that the symbol is in superscript in the first place? (I.e., instead of {}^{\\prime\\prime} it could be {\\prime\\prime}…)

(This may seem obscure, but actually it comes up surprisingly often when working with surface brightnesses, since they are traditionally something-per-square-arcsec)

[mhvk]

I guess the question here is whether we should catch 1/" as well - looking back at #9218, I see that we discussed it but decided to punt on it, for now only removing double superscripts.

[astropy-bot[bot]]

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