Update build-backend to scikit-build-core

[lgarrison]

This PR updates the PEP 517 build backend from classic scikit-build to modern scikit-build-core. Editable installs weren't working anymore in classic skbuild with modern setuptools, so it seemed best to just port to the modern backend.

I followed this migration guide, and it was all pretty straightforward. I had to bump some version requirements and tweak a few things, but for the most part it was a one-to-one transformation. Both regular and editable installs pass the tests now. Also, both types of installs now put the compiled CPython extension in the package directory in site-packages, rather than the source tree, which seems desirable.

I did notice that OpenMP isn't being used, though; is that intended? Is Jax going to do a higher-level parallelization?

Next steps (for upcoming PRs) are:

• Update the pinned version of finufft to use the latest version with proper CMake support
• Try to get jax-finufft to use the GPU interface that is now a part of finufft (rather than the separate cufinufft package, which is deprecated)

[dfm]

I did notice that OpenMP isn't being used, though; is that intended? Is Jax going to do a higher-level parallelization?

You mean we're not currently compiling with OpenMP support? I don't really remember looking into that. Will the current version of finufft with CMake support find it automatically?

[lgarrison]

You mean we're not currently compiling with OpenMP support? I don't really remember looking into that. Will the current version of finufft with CMake support find it automatically?

That's right; at least as I'm compiling it, I get lots of warnings about unknown OpenMP pragmas. We'll see if the new finufft turns it on by default; we obviously want it if Jax isn't doing its own parallelization!