Update serpapi.py Support baidu list type answer_box (#6386)
Support baidu list type answer_box
From [this document](https://serpapi.com/baidu-answer-box), we can know
that the answer_box attribute returned by the Baidu interface is a list,
and the list contains only one Object, but an error will occur when the
current code is executed.
So when answer_box is a list, we reset res["answer_box"] so that the
code can execute successfully.
