Use poetry-core build system

[mgorny]

Switch from the obsolete poetry build backend to poetry-core. They are functionally equivalent, except that the latter pulls much fewer dependencies in the virtualenv used to build wheels. See: https://python-poetry.org/docs/pyproject/#poetry-and-pep-517

[mgorny]

The documentation says to make a PR against develop branch but there doesn't seem to be such a branch.

[mgorny]

Ah, so apparently this package uses an undocumented build thing that works only with poetry… :-(.

[cdgriffith]

Never knew this, thanks for the links, new to using poetry, will try to switch!