Fixes related to the attachments module and Python3

We saw that the attachments 
* Fixed the filename generation when ``UPLOAD_PATH_OBSCURIFY`` setting is enabled. This uses the same method as my previsouly merged pull request, https://github.com/benjaoming/django-wiki/pull/254, which is compatible with both py2 and py3.
* Fixed the send_file method to correctly send filenames under Python3 (else the sent file will have a name like ``b'original_filename.pdf'``). This uses six to determine the Python version, so it is backwards-compatible with py2.