Use poetry-core build backend (#589)

Replace the deprecated poetry.masonry.api build backend with the more
modern poetry.core.masonry.api that avoids unnecessarily installing
whole poetry when building wheels.  The rationale is provided
on the project page: https://pypi.org/project/poetry-core/

pyproject.toml

@@ -1,6 +1,6 @@
 [build-system]
-requires = ["poetry>=0.12"]
-build-backend = "poetry.masonry.api"
+requires = ["poetry-core"]
+build-backend = "poetry.core.masonry.api"
 
 
 [tool.regarding]