DEP: Deprecate size-one ragged array coercion

Previously, code such as:
```
np.array([0, np.array([0])], dtype=np.int64)
```
worked by discovering the array as having a shape of `(2,)` and
then using `int(np.array([0]))` (which currently succeeds).
This is also a ragged array, and thus deprecated here earlier on.
(As a detail, in the new code the assignment should occur as
an array assignment and not using the `__int__`/`__float__`
Python protocols.)

Two details to note:

1. This still skips the deprecation for sequences which are not
   array-likes.  We assume that sequences will always fail the
   conversion to a number.
2. The conversion to a number (or string, etc.) may still fail
   after the DeprecationWarning is given.  This is not ideal, but
   narrowing it down seems tedious, since the actual assignment
   happens in a later step.
   I.e. `np.array([0, np.array([None])], dtype=np.int64)` will give
   the warning, but then fail anyway, since the array cannot be
   assigned.

Closes gh-16939

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