Django-delayed-union

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* Fixed setup.py metadata

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* Added support for Django 3.2 and 4.0.
* Dropped Python 2 support.

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* Added support for Python 3.8 and Django 3.0

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* Added query property to delayed querysets.
* Fixed bug with count() and select_related() in MySQL
* Added tests for Django 3.0

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* Added tests for Django 2.2

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* Added support for nested unions and intersections