Update build-system requirement to poetry-core

Build system was changed in ddbe061
to use poetry-core, but pyproject.toml still depends on the whole
poetry

pyproject.toml

@@ -108,5 +108,5 @@ profile = "black"
 
 [build-system]
 
-requires = ["poetry>=0.12"]
+requires = ["poetry-core"]
 build-backend = "poetry.core.masonry.api"