Use poetry-core as build-backend

[FRidh]

The build backend of poetry was split into a separate package called poetry-core.

Please update your pyproject.toml to use

[build-system]
requires = ["poetry-core"]
build-backend = "poetry.core.masonry.api"

For more info, see https://github.com/python-poetry/poetry-core#poetry-core.

[supakeen]

Thanks for letting me know, this will be fixed in the soon-to-be-released v1.2.3.