Use poetry-core build backend (#232)

https://github.com/python-poetry/poetry-core#why-is-this-required

Author: mweinelt

pyproject.toml

@@ -70,5 +70,5 @@ default_section = "THIRDPARTY"
 profile = "black"
 
 [build-system]
-requires = ["poetry>=1.1.12"]
-build-backend = "poetry.masonry.api"
+requires = ["poetry-core"]
+build-backend = "poetry.core.masonry.api"