fix(packaging): load the build backend from the tree

`backend-path = ["."]` with `build-backend = "setuptools.build_meta"` is
rejected by pip (BackendUnavailable), because PEP 517 requires a backend
named with backend-path to live in that path, breaking pip installs from a
checkout, Git URL, or sdist. uv does not enforce this. Make _build_hooks the
backend by re-exporting setuptools.build_meta, per setuptools' documented
in-tree backend pattern.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: AbhiPrasad

py/_build_hooks.py

@@ -1,7 +1,12 @@
-"""Setuptools build hooks: keep colocated tests out of the wheel."""
+"""In-tree PEP 517 backend: setuptools, minus colocated tests in the wheel.
+
+pyproject.toml loads this module via ``backend-path``, which requires the
+backend itself to live in the tree, so it re-exports ``setuptools.build_meta``.
+"""
 
 import fnmatch
 
+from setuptools.build_meta import *  # noqa: F403
 from setuptools.command.build_py import build_py as _build_py
 
 

py/pyproject.toml

@@ -1,6 +1,6 @@
 [build-system]
 requires = ["setuptools>=82.0.1"]
-build-backend = "setuptools.build_meta"
+build-backend = "_build_hooks"
 backend-path = ["."]
 
 [project]