系统转移过程中，擅自把aptitude安装的mongoengine换成了pip安装，系统启动以后，报这个错误
报错提示：
File "/usr/local/lib/python2.7/dist-packages/pymongo/mongo_client.py", line 748, in _socket_for_reads
with self._get_socket(read_preference) as sock_info:
File "/usr/lib/python2.7/contextlib.py", line 17, in __enter__
return self.gen.next()
File "/usr/local/lib/python2.7/dist-packages/pymongo/mongo_client.py", line 712, in _get_socket
server = self._get_topology().select_server(selector)
File "/usr/local/lib/python2.7/dist-packages/pymongo/topology.py", line 141, in select_server
address))
File "/usr/local/lib/python2.7/dist-packages/pymongo/topology.py", line 117, in select_servers
self._error_message(selector))
ServerSelectionTimeoutError: No servers found yet
因为是系统转移，刚开始是以为mongodb服务器那边服务的问题，后来上网查了一下，发现是pymongo的3.0.*以后版本接口向前兼
