Fix const-correctness of iterators in cypclass set

Cython/Includes/libcythonplus/set.pxd

@@ -204,7 +204,7 @@ cdef cypclass cypset[V]:
 
     cypset[V] __ior__(self, const cypset[V] other) except ~:
         if self._active_iterators == 0:
-            self._elements.insert(other._elements.const_begin(), other._elements.end())
+            self._elements.insert(other._elements.const_begin(), other._elements.const_end())
             return self
         else:
             with gil: