Add NumPy 2 support

[cbrnr]

Fixes #493.

[cbrnr]

@tompollard you opened #506, #507, and #508 – do the changes in this PR not solve the issue? All tests pass with NumPy 2.

[cbrnr]

This could even remain numpy = ">=1.26.0", since all changes should work with both NumPy 1.x and NumPy 2.x.

[tompollard]

@cbrnr thanks for looking at this and sorry for the duplicate effort.

@tompollard you opened #506, #507, and #508 – do the changes in this PR not solve the issue? All tests pass with NumPy 2.

I think the change in this pull request resolves one of the issues, but not all of them. Errors may not be obvious because (1) the test suite doesn't have full coverage and (2) overflows don't raise errors by default.

I have tried to address the warnings raised when running tests with NPY_PROMOTION_STATE=weak_and_warn, but I think more work is needed (e.g. by extending tests).

We may want to set the package to raise error on overflow when in development, which I think we can do with np.seterr (https://numpy.org/doc/stable/reference/generated/numpy.seterr.html)

[cbrnr]

No problem! Feel free to take whatever you need from this PR, and let me know if you need further help. It would be great if NumPy 2 support wasn't too far in the future!