Only build with Cython alpha on the free-threaded build

packaging/pep517_backend/_backend.py

@@ -4,6 +4,7 @@
 from __future__ import annotations
 
 import os
+import sysconfig
 import typing as t
 from contextlib import contextmanager, nullcontext, suppress
 from functools import partial
@@ -371,10 +372,12 @@ def get_requires_for_build_wheel(
             stacklevel=999,
         )
 
-    c_ext_build_deps = [] if is_pure_python_build else [
-        'Cython == 3.1.0a1; python_version >= "3.13"',
-        'Cython ~= 3.0.0; python_version <= "3.12"',
-    ]
+    if is_pure_python_build:
+        c_ext_build_deps = []
+    elif sysconfig.get_config_var("Py_GIL_DISABLED"):
+        c_ext_build_deps = ['Cython == 3.1.0a1']
+    else:
+        c_ext_build_deps = ['Cython ~= 3.0.0']
 
     return _setuptools_get_requires_for_build_wheel(
         config_settings=config_settings,