Fix pyproject.toml build backend (setuptools.build_meta)

setuptools.backends.legacy is not available on the runner's setuptools
version. The standard build backend is setuptools.build_meta.

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

Author: wasade

pyproject.toml

@@ -1,6 +1,6 @@
 [build-system]
 requires = ["setuptools>=64", "setuptools-scm>=8"]
-build-backend = "setuptools.backends.legacy:build"
+build-backend = "setuptools.build_meta"
 
 [project]
 name = "microsetta-interface"