pyproject: switch use non-legacy setuptools build backend

This may break some build configurations, like pip. But we should be
using uv everywhere now. I don't have the patience to deal with pip
anymore.

Author: indygreg

docs/news.rst

@@ -25,6 +25,8 @@ Version History
   tests against a separate build that was theoretically identical. But the
   builds may have been subtly different, leading to preventable bugs in our
   wheels. (Enabling this test coverage did not uncover any failures.)
+* The ``pyproject.toml`` build backend has been switched from
+  ``setuptools.build_meta:__legacy__`` to ``setuptools.build_meta``.
 
 0.24.0 (released 2025-08-17)
 ============================

pyproject.toml

@@ -38,8 +38,7 @@ requires = [
     "packaging",
     "setuptools",
 ]
-# Need to use legacy backend because setup_zstd.py breaks build isolation.
-build-backend = "setuptools.build_meta:__legacy__"
+build-backend = "setuptools.build_meta"
 
 [dependency-groups]
 dev = [