MAINT: make __str__ less ambiguous

[ev-br]

For 0D arrays, __str__ used to look confusingly like a scalar, make it clearly an array instead.

Before:

In [1]: import array_api_strict as xp

In [2]: x = xp.asarray(3)

In [3]: print(x)     # looks like a scalar!
3

In [4]: x.__class__
Out[4]: array_api_strict._array_object.Array

In [5]: x
Out[5]: Array(3, dtype=array_api_strict.int64)

This PR:

In [1]: import array_api_strict as xp

In [2]: x = xp.asarray(3)

In [3]: print(x)
Array(3, dtype=array_api_strict.int64)

In [4]: x
Out[4]: Array(3, dtype=array_api_strict.int64)