docker.from_env blocks with paramiko on python2.7

The code below block always reproduces with python2.7 on MacOS when DOCKER_HOST=ssh://somehost. The same code works fine with on other python versions and I also tested with latest paramiko==2.5.0. This seems to be related to an ancient paramiko ticket at https://github.com/paramiko/paramiko/issues/109 which could be avoided based on few comments I read there.

exitstatus: 2
    client = docker.from_env(timeout=5, version="auto")
../../.pyenv/versions/2.7.16/lib/python2.7/site-packages/docker/client.py:85: in from_env
terminalreporter: <_pytest.terminal.TerminalReporter object at 0x109c54ed0>
    timeout=timeout, version=version, **kwargs_from_env(**kwargs)
../../.pyenv/versions/2.7.16/lib/python2.7/site-packages/docker/client.py:40: in __init__
    self.api = APIClient(*args, **kwargs)
../../.pyenv/versions/2.7.16/lib/python2.7/site-packages/docker/api/client.py:166: in __init__
    base_url, timeout, pool_connections=num_pools
../../.pyenv/versions/2.7.16/lib/python2.7/site-packages/docker/transport/sshconn.py:84: in __init__
    self._connect()
../../.pyenv/versions/2.7.16/lib/python2.7/site-packages/docker/transport/sshconn.py:94: in _connect
    parsed.hostname, parsed.port, parsed.username,
../../.pyenv/versions/2.7.16/lib/python2.7/site-packages/paramiko/client.py:397: in connect
    t.start_client(timeout=timeout)
../../.pyenv/versions/2.7.16/lib/python2.7/site-packages/paramiko/transport.py:597: in start_client
    event.wait(0.1)
../../.pyenv/versions/2.7.16/lib/python2.7/threading.py:614: in wait
    self.__cond.wait(timeout)
../../.pyenv/versions/2.7.16/lib/python2.7/threading.py:359: in wait
    _sleep(delay)
E   KeyboardInterrupt

Apparently one problem is that the ssh connect lacks passing the timeout parameter which makes it blocking, something for which I already opened a PR.

The second issue is why this haappens, which is likely a bug in paramiko as the same code works fine with other versions of Python on the same machine.

Start with docker/transport/sshconn.py, especially the _connect path shown in the traceback, and reproduce the block with Python 2.7 and DOCKER_HOST=ssh://somehost. Compare the SSH connection behavior across the reported Python and Paramiko versions; done means the reproduction no longer blocks or the compatibility limitation is clearly established.