array API standard: return an array for `sparse.sum` of a 2-D array

[lucascolley]

I don't see this missing coverage indicated in https://github.com/pydata/sparse/blob/main/ci/Numba-array-api-xfails.txt, maybe there isn't a test for this?

(Pdb) import sparse as xp
(Pdb) xp.sum(xp.eye(2))
np.float64(2.0)