Add scikit-build-core

[henryiii]

I picked bad timing for this, as it looks like the next version of mypy has issues with it:

old
> /tmp/mypy_primer/mypy_old/venv/bin/mypy src tests noxfile.py --python-executable=/tmp/mypy_primer/projects/_scikit-build-core_venv/bin/python --warn-unused-ignores --warn-redundant-casts --no-incremental --cache-dir=/dev/null --show-traceback --soft-error-limit=-1 (27.8s)
	Success: no issues found in 165 source files
----------

new
> /tmp/mypy_primer/mypy_new/venv/bin/mypy src tests noxfile.py --python-executable=/tmp/mypy_primer/projects/_scikit-build-core_venv/bin/python --warn-unused-ignores --warn-redundant-casts --no-incremental --cache-dir=/dev/null --show-traceback --soft-error-limit=-1 (27.9s)
	src/scikit_build_core/settings/sources.py:498: error: "DataclassInstance" not callable  [operator]
	src/scikit_build_core/settings/sources.py:587: error: Argument 1 to "convert_target" of "SourceChain" has incompatible type "Union[Type[DataclassInstance], DataclassInstance]"; expected "Type[DataclassInstance]"  [arg-type]
	src/scikit_build_core/settings/documentation.py:88: error: Unused "type: ignore" comment  [unused-ignore]
	src/scikit_build_core/settings/documentation.py:105: error: Argument 1 to "mk_docs" has incompatible type "Union[Type[DataclassInstance], DataclassInstance]"; expected "Type[object]"  [arg-type]
	src/scikit_build_core/settings/documentation.py:111: error: Argument 1 to "mk_docs" has incompatible type "Union[Type[DataclassInstance], DataclassInstance]"; expected "Type[object]"  [arg-type]
	src/scikit_build_core/file_api/reply.py:98: error: Argument 2 to "make_class" of "Converter" has incompatible type "Union[Type[DataclassInstance], DataclassInstance]"; expected "Type[DataclassInstance]"  [arg-type]
	src/scikit_build_core/settings/json_schema.py:117: error: Argument 1 to "to_json_schema" has incompatible type "Union[DataclassInstance, Type[DataclassInstance]]"; expected "Type[Any]"  [arg-type]
	Found 7 errors in 4 files (checked 165 source files)

Not sure what the policy is for projects that were error free in the last released version, I can wait until after release if that's better. Looks like it's not far away: python/mypy#18739.

Edit: I worked around most of the issues.

[hauntsaninja]

It's fine for it to not pass cleanly, the diff logic will handle that!