django-q API will change

Hi,

As part of python3.7 release, async became a reserved word, and django-q got fixed. All async_* functions got renamed async_task_* and the Async class got renamed AsyncTask.

I saw an impact only on tasks.py, where Async is imported. I'll probably make a Debian patch for the current release of HyperKitty, but maybe you should fix it. :)

Cheers!