Even after #30580, setup.py still has an import-time dependency on Cython (via sage_setup).

We declare this build system dependency by adding the PEP 517 metadata (pyproject.toml).

Adding pyproject.toml does not change how the Sage distribution installs sagelib because build/pkgs/sagelib/spkg-install uses setup.py install directly.

References:

• https://snarky.ca/what-the-heck-is-pyproject-toml/

CC: @tobiasdiez @jhpalmieri @dimpase

Component: build

Keywords: sd111

Branch/Commit: u/mkoeppe/pyproject_toml @ bbfc19e

Issue created by migration from https://trac.sagemath.org/ticket/30581