Change build backend

The introduction of REUSE compatibility breaks the build of the wheel
file.  python-poetry/poetry-core#57 closed the
issue on the poetry side, although `poetry-core` has not yet been
updated.  Nevertheless, I still do this change now, to have it available
for future releases.

pyproject.toml

@@ -125,5 +125,5 @@ testpaths = [
 python_classes = ''
 
 [build-system]
-requires = ["poetry>=0.12"]
-build-backend = "poetry.masonry.api"
+requires = ["poetry-core"]
+build-backend = "poetry.core.masonry.api"