... as a basis for a PEP 517 build system (#29845).

• https://pypi.org/project/pytoml/ is not actively maintained, but replacing it with something else is being discussed at pytoml is deprecated. Repalce with toml pypa/flit#255

• https://pypi.org/project/flit-core/

• https://pypi.org/project/flit/

flit_core installs itself using PEP 517, so this depends on the pip update in #29803; and we also have to fix sage-pip-install, which assumes a setuptools/distutils package.

tarballs: see checksums.ini

Depends on #31118

Component: build

Author: Matthias Koeppe

Branch/Commit: u/mkoeppe/add_packages_flit_and_dependencies_flit_core__pytoml @ 2326a6a

Issue created by migration from https://trac.sagemath.org/ticket/29846