Fix build with numpy v2

[t20100]

This PR proposes 2 changes:

• Use cimport numpy as cnp instead of np to differentiate import from cimport as suggested in https://cython.readthedocs.io/en/latest/src/tutorial/numpy.html#adding-types
• Replace the use of np.int_t type by long to restore the typed defined in numpy v1: see https://github.com/numpy/numpy/blob/v1.26.5/numpy/__init__.pxd#L258 and https://github.com/numpy/numpy/blob/v1.26.5/numpy/__init__.pxd#L712).

closes #239

[tschoonj]

Thanks for opening the PR, I will take a look at this soon

[t20100]

CI failure on macos may be related to the change of macos-latest which is now macos-14 which runs on arm64.

[tschoonj]

@t20100 I think this is ready for a merge now?

I have deployed your patch to Conda-forge already, so conda users will be able to use numpy 2 already.

[t20100]

I think this is ready for a merge now?

It's OK for me!

Thanks for already having deployed the fix on conda!