SubfieldBase has been deprecated. Use Field.from_db_value

[ojengwa]

We need to update this module for future Django release to fix this warning:

RemovedInDjango110Warning: SubfieldBase has been deprecated. Use Field.from_db_value instead.
  class JSONField(six.with_metaclass(models.SubfieldBase, models.Field)):

I will send in a PR.