Migration to scipy.sparse array API?

[kosack]

Scipy 15+ recommends using the new sparse array functions instead of sparse matrices. (e.g. scipy.sparse.csr_array instead of scipy.sparse.csr_matrix).

https://docs.scipy.org/doc/scipy-1.15.0/reference/sparse.migration_to_sparray.html#migration-to-sparray

Not critical now, but something we should maybe remember to do in case they deprecate the old way.