Hi Richard,
Django has fully supported Python 3 for a while now, see this talk:
Also IMHO it's less a matter of which users use Python 3 (I'd say it's likely most serious Python users mix-and-match on a project-to-project basis, depending on the age of the original code, but use Python 3 where they can).
I'd say it's a case of which hosting servers are running Python 3 at the time of deployment.
Often the default version of Python supported by hosts is 2.7 or even 2.6, so it's a matter of whether the costs outweigh the benefits of installing and maintaining your own versions of Python 3.