pyproject.toml: use poetry-core backend for PEP517

[benkohler]

Per poetry upstream, "If your pyproject.toml file still references poetry directly as a build backend, you should update it to reference poetry-core instead."

https://python-poetry.org/docs/pyproject/#poetry-and-pep-517 https://projects.gentoo.org/python/guide/distutils.html#deprecated-pep-517-backends

Signed-off-by: Ben Kohler [email protected]