Fix build backend: use setuptools.build_meta instead of legacy path

setuptools.backends.legacy:build is not supported by the pip vendor
bundled with Python 3.14. Classic setuptools.build_meta works cleanly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pyproject.toml

@@ -1,6 +1,6 @@
 [build-system]
 requires = ["setuptools>=68", "wheel"]
-build-backend = "setuptools.backends.legacy:build"
+build-backend = "setuptools.build_meta"
 
 [project]
 name = "deceit_env"