dockerpty depends on docker-py even when it doesn't import it. This library imports docker-py only in tests, otherwise it expects to get Client instance. The only `import docker` in code is in this commentary:

```
class PseudoTerminal(object):
    """
    Wraps the pseudo-TTY (PTY) allocated to a docker container.

    The PTY is managed via the current process' TTY until it is closed.

    Example:

        import docker
```

Can we remove the dependency then?