be a little more conservative in the last change

Cython/Compiler/ExprNodes.py

@@ -2002,6 +2002,7 @@ class SliceIndexNode(ExprNode):
     def analyse_target_types(self, env):
         self.analyse_types(env)
         # when assigning, we must accept any Python type
+        if self.type.is_pyobject:
             self.type = py_object_type
 
     def analyse_types(self, env):