Printing Promise objects from asynchronous Runner.run() raises Attribute Error

[dqian3]

@task
def test(c):
    promise = c.run("sleep 5", asynchronous=True)
    print(promise)
    promise.join()

As a new user of invoke, I noticed that when I would try to print the promise returned by a Runner.run call with asynchronous=True (see above code for an example), I would get the following error:

AttributeError: 'Promise' object has no attribute 'exited'

Looking at runners.py, it seems the issue is that since Promise is a subclass of Result, and so inherits its __str__ method.