Support for django v3

[bakonyidani]

I am currently trying to update to using django 3.0.3, but django-annoying throws an error:

 File "/home/dani/anaconda3/envs/artheseus/lib/python3.7/site-packages/annoying/fields.py", line 11, in <module>
    from django.utils import six
ImportError: cannot import name 'six' from 'django.utils' (/home/dani/anaconda3/envs/artheseus/lib/python3.7/site-packages/django/utils/__init__.py)

Indedd django.utils.six. was removed in version 3:
django/django@4138481

[skorokithakis]

Hmm, yes, that's true. Would you mind issuing a PR with a conditional import that works on 3?

[ZeroCoolHacker]

I believe its done so the issue should be closed now

[skorokithakis]

Good call, thanks!