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* Fix deferred attribute for Django >=2.1, <3.0
* Django: Drop support for [1.11, 2.0], add support for [2.2, 3.0, 3.1]
* Python: Drop support for [2.7, 3.5], add support for [3.6, 3.7, 3.8]
* Replace force_text by force_str (deprecated in Django 4.0)
* Improved test coverage
* Support for Django ModelFields