Support numpy 2.0

[kbattocchi]

numpy 2.0 has been released, which has breaking changes with numpy 1.*. However, at least in theory it should be possible to support both numpy 1 and numpy 2 at runtime if wheels are built with numpy 2.

For the most part, this will entail replacing oldest-supported-numpy with numpy>=2 in the build-system/requires portion of pyproject.toml; however we should only do this for python_version>='3.9', since numpy 2 will not be released on python 3.8 and we're still supporting that for now.

However, there may also be changes required to our native tree code, since that relies on parts of the numpy API that may have had backwards-incompatible changes.

[rootsmusic]

Tracking at numpy/numpy#26191