Build with the poetry-core backend, the legacy poetry one is gone.

diff -Nrup a/pyproject.toml b/pyproject.toml
--- a/pyproject.toml	2023-07-23 10:57:02.000000000 +0200
+++ b/pyproject.toml	2026-10-22 17:22:16.476920719 +0200
@@ -32,4 +32,4 @@ matrix_upload = ["python-magic", "reques
 
 [build-system]
 requires = ["poetry>=0.12"]
-build-backend = "poetry.masonry.api"
+build-backend = "poetry.core.masonry.api"