units and constants and NEP 42

@mhvk co-authored [NEP 42](https://numpy.org/neps/nep-0042-new-dtypes.html) and it is accepted by Numpy. How will this affect `astropy.units` and `astropy.constants`? Do they need a complete refactor? Is NEP 42 sufficient for all the weird unit conversions that astronomers do (looking at you, magnitude systems)?

cc @adrn @nstarman @mhvk @stargaser

Start by reading NEP 42 and the astropy.units and astropy.constants entry points named in the issue. Compare NEP 42's capabilities with astronomical unit conversions, including magnitude systems; done means documenting the impact, limitations, and whether a refactor is needed.

Written by the indexing model from the issue text.