ssh_config `ProxyJump none` is not handled correctly

Describe the bug Setting the ssh_config ProxyJump option to the value none should deactivate the feature (and prevent later entries in the config from applying). This is currently not the case, none is treated like a hostname, breaking the connection.

Ref: https://man.openbsd.org/OpenBSD-current/man5/ssh_config.5#ProxyJump

To Reproduce Steps to reproduce the behaviour (please attach a minimal example):

1. Setup a ssh_config with a Host that includes ProxyJump none
2. python复制

   from fabric import Connection
   
   conn = Connection("host")
   if conn.gateway.host == "none":
       print("This should not happen")

Expected behaviour Just like when using a normal ssh client, I expect ProxyJump none to disable the proxy jumping.