mqtt_client = MQTTClient('uf2', 'y90166f4.ala.cn-hangzhou.emqxsl.cn', port=8883, user='test', password='test', keepalive=0, ssl=True, ssl_params={"cert":"/flash/certificate/emqxsl-ca.crt", "server_hostname":'y90166f4.ala.cn-hangzhou.emqxsl.cn'})

ertfile /flash/certificate/emqxsl-ca.crt
keyfile None
Traceback (most recent call last):
  File "<stdin>", line 31, in <module>
  File "<stdin>", line 23, in setup
  File "umqtt/simple.py", line 1, in connect
  File "ssl.py", line 1, in wrap_socket
  File "ssl.py", line 1, in load_cert_chain
TypeError: can't convert 'NoneType' object to str implicitly