Enable building documentation under Python 3

This is a small fix which I have on my own local copy. I'm proposing the PR only because others may find it useful if they build the documentation under Py3.

Currently, if you build the tawhiri documentation under Py3, sphinx barfs[0] at download.py because it uses the three argument raise which is syntactically invalid in Python 3. This PR uses six's reraise() function which is exactly equivalent to the three-argument raise in Python 2[1] and Does The Right Thing(TM) in Python 3. It's behaviour in Python 3 is not of interest, however, since the downloader is Python 2.

The rationale for this change is to allow download.py to be *syntactically* correct Python 3 meaning that sphinx's autodoc will at least parse the file.

I am aware that adding a dependency on six is slightly annoying and that touching download.py at all is dangerous and so no ego will be bruised if this isn't merged. I just got sick of seeing the parse errors. Perhaps we should have a ``ocpd`` branch? :smile: 

[0] But does continue. Documentation will still be built. The error is annoying, though.
[1] Modulo adding another function call to the traceback in some cases.