Djfernet

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Switched back to django-fernet-fields for default salt, making it incompatible
with 0.7.4! But, compatible with django-fernet-fields, so that you can migrate
easily if you haven't already.

If, unfortunnately, you have already deployed this in production, you have two
options:

- re-encrypt your data to use django-fernet-fields instead of djfernet,
- or set settings.DJFERNET_PREFIX=djfernet to keep going

Sorry about this laborious releases.

Also, added EncryptedBinaryField.

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First release since fork. Unfortunnately, my sed
s/django-fernet-fields/djfernet/ caused a change in the salt, make it
impossible to decrypt existing data!!

Added settings.DJFERNET_PREFIX to set it to django-fernet-fields and make it
compatible again through a setting.

Thanks to sevdog for the report.

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* Support Postgres 10
* Drop support for Django < 1.11, Python 3.3/3.4
* Add support for Django 1.11 through 2.2, Python 3.7

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* Support Django 1.10.

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* Add `isnull` lookup.

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* Remove DualField and HashField. The only cases where they are useful, they
aren't secure.