Remove `python/build-requirements.txt`

[jhale]

Describe new/missing feature

scikit-build-core now supports extracting build requirements from pyproject.toml:

python -m scikit_build_core.build requires

giving:

[
  "mpi4py",
  "nanobind>=2.5.0",
  "scikit-build-core[pyproject]>=0.10"
]

This could be used to remove build-requirements.txt, although it would require some intermediate text processing to be compatible with pip.

Suggested user interface