relax cython version spec

Index: pyproject.toml
--- pyproject.toml.orig
+++ pyproject.toml
@@ -16,7 +16,7 @@
 requires = [
     "setuptools>=77.0.1",
     "wheel",
-    "Cython==3.1.1",
+    "Cython>=3.1.1",
 ]
 build-backend = "setuptools.build_meta"
 
