Add support for numpy 2

[aphearin]

In #1083 the dependency chain added a requirement numpy<2. The reason was that none of our CI was running due to some unknown incompatibility with numpy 2. So we kicked the can down the road on support for numpy 2, but this should be restored once numpy 2 incompatibility causes issues with common environments.

[aphearin]

According to this failed job log, one of the issues with the cython has to do with how cython arrays of integer type are declared:

cdef vector[np.int_t] j_ind

The Numpy 2.0 migration guide has a few comments about this here.

In addition to changing the way integer arrays are declared, a separate fix using the import_array() mechanism may also be required according to this section of the migration guide.