Reduce compiler warnings and pin scikit-build-core

Minor changes to reduce the number of compiler warnings.

• Restore the pre-compiler define NPY_NO_DEPRECATED_API=0 we had in the previous build system, with setup.py.
• Remove deprecated CPython functions, which do nothing since Python 3.9: PyEval_InitThreads() and PyEval_ThreadsInitialized().
• Gives names to some empty enum objects (forward declarations).
• Fix "shadows a member of" warnings in extension.

Added an extra test to for groups, found when trying to remove the warning about deprecated std::auto_ptr. A solution wasn't found for that (neither std::shared_ptr nor boost::shared_ptr didn't work as a replacement.

(Still around 4200 warnings, down from 4450, when building with pedantic warnings!)

Pin scikit-build-core

With the newly-released version 0.6.0, we get an import error with pip install -e .. Trying to work around that caused strange circular import failures.