--- a/build_tools/cythonize.py
+++ b/build_tools/cythonize.py
@@ -59,7 +59,7 @@
     except ImportError:
         pass
 
-    flags = ['--fast-fail']
+    flags = ['--fast-fail', '-3']
     if gen_file.endswith('.cpp'):
         flags += ['--cplus']