Django 3.0 support

[glanzel]

I had this error:

File "/home/username/.local/lib/python3.7/site-packages/django_baker/bakery.py", line 10, in <module>
    from django.utils.six import iteritems
ModuleNotFoundError: No module named 'django.utils.six'

i changed it into
from six import iteritems
and now it works.

[opencoca]

Can confirm that this fixes things.

Also in admin.py from django.core.exceptions import FieldDoesNotExistneeds to replace from jango.db.models.fields import FieldDoesNotExist

[opencoca]

Please see and approve #26

[opencoca]

I've taken to reviewing pull requests to @krisfields django-baker and mannually mirroring them to my https://github.com/opencoca/django-baker