python用requests库给https发请求报ssl异常，请大神帮忙进来看下，谢谢！

代码如下 证书文件是有效的

headers = {'Content-Type': 'application/json'}
response = requests.post(url, data=data, headers=headers, cert=('./credoo_stg.crt','./credoo_stg.key'),verify=True)


Enter PEM pass phrase:
Traceback (most recent call last):
File "test.py", line 31, in <module>
response = requests.post(url, data=data, headers=headers, cert=('./credoo_stg.crt','./credoo_stg.key'),verify=True)
File "/usr/local/lib/python2.7/site-packages/requests/api.py", line 109, in post
return request('post', url, data=data, json=json, **kwargs)
File "/usr/local/lib/python2.7/site-packages/requests/api.py", line 50, in request
response = session.request(method=method, url=url, **kwargs)
File "/usr/local/lib/python2.7/site-packages/requests/sessions.py", line 465, in request
resp = self.send(prep, **send_kwargs)
File "/usr/local/lib/python2.7/site-packages/requests/sessions.py", line 573, in send
r = adapter.send(request, **kwargs)
File "/usr/local/lib/python2.7/site-packages/requests/adapters.py", line 431, in send
raise SSLError(e, request=request)
requests.exceptions.SSLError: [SSL: UNKNOWN_PROTOCOL] unknown protocol (_ssl.c:590)