Do you think this is some kind of bug in six, or am I missing something? Shouldn't `reraise` behave the same way for both python versions?

I had to use this if clause here or else I would get green tests in python 2 and the following error in python 3:

```
Traceback (most recent call last):
  File "/home/tassadar/code/github/unstdlib.py/test/test_standard.py", line 47, in test_convert_exception
    throw_foo()
  File "/home/tassadar/code/github/unstdlib.py/unstdlib/standard/exception_.py", line 42, in fn_new
    reraise(new_exception, None, traceback)
  File "/home/tassadar/code/github/unstdlib.py/unstdlib/six.py", line 489, in reraise
    if value.__traceback__ is not tb:
AttributeError: 'NoneType' object has no attribute '__traceback__
```