code: def _load_ssh_auth_post_yosemite(mac_username):
      
      user_id = subprocess.check_output(['id', '-u', mac_username])
      ssh_auth_sock = subprocess.check_output(['launchctl', 'asuser', user_id, 'launchctl', 'getenv', 'SSH_AUTH_SOCK']).rstrip()
      _set_ssh_auth_sock(ssh_auth_sock)
docs: Starting with Yosemite, launchd was rearchitected and now only one
      launchd process runs for all users. This allows us to much more easily
      impersonate a user through launchd and extract the environment
      variables from their running processes.