python3Packages.arch: fix build failure

pkgs/development/python-modules/arch/default.nix

@@ -1,6 +1,7 @@
 {
   lib,
   buildPythonPackage,
+  python,
   fetchFromGitHub,
   cython,
   numpy,
@@ -8,9 +9,9 @@
   property-cached,
   pytestCheckHook,
   scipy,
-  setuptools,
   setuptools-scm,
   statsmodels,
+  meson-python,
 }:
 
 buildPythonPackage rec {
@@ -25,17 +26,10 @@ buildPythonPackage rec {
     hash = "sha256-qw8sSgsMu6YTiQlzsrePnDKkFBtrxD9RK6ZZE5jFeX0=";
   };
 
-  postPatch = ''
-    substituteInPlace setup.cfg \
-      --replace-fail 'PytestRemovedIn8Warning' 'PytestRemovedIn9Warning'
-    substituteInPlace pyproject.toml \
-      --replace-fail '"setuptools_scm[toml]>=8.0.3,<9",' '"setuptools_scm[toml]",'
-  '';
-
   build-system = [
-    setuptools
     setuptools-scm
     cython
+    meson-python
   ];
 
   dependencies = [
@@ -48,6 +42,16 @@ buildPythonPackage rec {
 
   nativeCheckInputs = [ pytestCheckHook ];
 
+  # Replace the source tree's arch/ with symlinks to the installed package.
+  # pytest adds rootdir to sys.path, so the source arch/ always shadows the
+  # installed one. The source tree lacks _version.py (generated by setuptools-scm)
+  # and compiled Cython extensions, causing import failures.
+  # Symlinking to the installed package makes all imports resolve correctly.
+  preCheck = ''
+    rm -rf arch
+    ln -s $out/${python.sitePackages}/arch arch
+  '';
+
   disabledTestPaths = [
     # Skip long-running/failing tests
     "arch/tests/univariate/test_forecast.py"