Add support for NumPy v2.0

[matt-graham]

We currently pin our NumPy dependency to <2:

s2fft/pyproject.toml

Lines 29 to 30 in 876e090

	dependencies = [
	"numpy>=1.20,<2",

This is problematic for users relying on other packages requiring NumPy 2 or above.

[matt-graham]

From a quick check of trying to install s2fft in a fresh Python 3.12 environment with the upper NumPy version constraint removed, it looks like a blocker for now is that there does not appear to be a NumPy v2.0 compataible pyssht release on PyPI - astro-informatics/ssht#75 should have made such a release possible, but it looks like the last release on PyPI was in March 2023.

[jasonmcewen]

Yes, see further discussion in PR 267 here.

Mayeul had been maintaining that but I don't think he's had time lately.

Do you know how do to that @matt-graham ?