six.wraps doesn't ignore non-existing attributes like Python 3 does

[immerrr]

Here's a repro script that fails in Python 2, but works in Python 3:

import six
class Hello(object):
    def __call__(self):
        return "Hello"
f = Hello()
@six.wraps(f)
def world():
    return f() + ", world"

On Python 2 it fails with

AttributeError: 'Hello' object has no attribute '__name__'

On Python 3 it succeeeds, because it was fixed in in https://bugs.python.org/issue3445. Given the trivial implementation it should be enough to backport the update_wrapper function as-is.

[immerrr]

This is probably a duplicate of #165