DEP: Deprecate coercion to subarray dtypes

When coercing to subarray dtypes, e.g. using `np.array(obj, dtype)`,
but also `arr.astype(dtype)`, the behaviour was only well defined
with tuple inputs, but not with array-like inputs.
In particular, `arr.astype(dtype)` had arguably surprising behaviour
of not converting by element, but rather attempting (and often failing)
to broadcast `arr` to the result array with added dimensions.

This deprecates all of these cases, the main issue would be for users
relying on stranger inputs with broadcasted tuples contained in
sequences:
```
np.array([((0, 1), (1, 2)), ((2,),)], dtype='(2,2)f4')
```

In most cases, where the tuples have the correct output shape,
the new base dtype can be directly used since the discovered shape
should match.
However, there is no work-around for the above case.

Closes gh-17173

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