python Requests 模拟登录知乎出现<Response [403]>?

以下是代码

import requests
from bs4 import BeautifulSoup as bs

zhihu='https://www.zhihu.com/#signin'

def login():
    header = {
        "Accept": "*/*",
        "Accept-Encoding": "gzip,deflate",
        "Accept-Language": "en-US,en;q=0.8,zh-TW;q=0.6,zh;q=0.4",
        "Connection": "keep-alive",
        "Content-Type": " application/x-www-form-urlencoded; charset=UTF-8",
        "User-Agent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/38.0.2125.111 Safari/537.36",
        "Referer": "http://www.zhihu.com/"
    }
    s = requests.session()
    cookie = bs(s.get('https://www.zhihu.com', headers=header).content).find(type='hidden')['value']
    print(cookie)
    post_data = {'_xsrf': cookie,  'password': 'xxx','captcha_type':'cn','remember_me': True,'email': 'xxxx'}
    p=s.post(url=zhihu, data=post_data, headers=header)
    print(p)
    r = s.get(url=zhihu)
   

if __name__ == '__main__':  # 程序运行入口
    login()

请问出现 403 错误的原因？