Move to scikit-build-core

[isuruf]

No description provided.

[isuruf]

This is ready for review. Note that this would require developers to use

pip install -e . --no-build-isolation

instead of

python setup.py develop

when developing symengine.

[bjodah]

This is great, I just learned of scikit-build-core the other week (they really should consider another name, it sounds like an internal package rather than the "modern" alternative to scikit-build).