Post-numpy-2.0 changes in repr/str #15793

What is the problem this feature will solve?

Numpy 2.0 is becoming much more explicit with how scalars are represented. In most cases, we can just adjust our docs - see #15065 - but some become decidedly ugly, and we should change our default __str__ and __repr__. Specific cases:

1. StructuredUnit. Some of its properties such as .physical_type return a np.void structure. This used to be represented as

((PhysicalType('length'), PhysicalType({'speed', 'velocity'})), PhysicalType('time'))

but with numpy 2.0, this becomes

astropy.units.structured.Structure((astropy.units.structured.Structure((PhysicalType('length'), PhysicalType({'speed', 'velocity'})), dtype=[('f0', 'O'), ('f1', 'O')]), PhysicalType('time')), dtype=[('f0', 'O'), ('f1', 'O')])

Which is not very useful. @nstarman's suggestion in #15065 (comment) is to make __str__ look like the old repr and create a new __repr__ that at the very least corrects the name to astropy.units.Structure. Since for those Structure, the dtype is always object, that information can be omitted as well, in which case it would become,

astropy.units.Structure((astropy.units.Structure((PhysicalType('length'), PhysicalType({'speed', 'velocity'})), PhysicalType('time')))

though perhaps this goes a bit too far, since now we have lost the names as well.

Describe the desired outcome

We have clean and informative __str__ and __repr__, the latter ideally sufficient to reproduce the item in question.

Additional context

No response