Fix

mypyc/lib-rt/list_ops.c

@@ -260,7 +260,7 @@ PyObject *CPyList_Extend(PyObject *o1, PyObject *o2) {
     if (res == -1) {
         return NULL;
     }
-    return o1;
+    return Py_None;
 }
 
 // Return -2 or error, -1 if not found, or index of first match otherwise.