[zahlman]

Python projects often need to share space with code written in other languages used for extensions - especially C traditionally, but now there are many competitors (and Numpy also uses Fortran for LAPACK and possibly also BLAS; I'd have to check).

PEP 518 gave the original definition for pyproject.toml contents, defining its use for describing build-time requirements. (PEP 517 appeared more or less in tandem, describing the hook to call an entry point for the build backend; later, PEP 621 standardized some metadata for the build backend to use.) Many other options were considered, including for the name: https://peps.python.org/pep-0518/#other-file-names