Drop support for Django 4.0 #581

Job	Run time
Python 3.7, Database sqlite	1m 6s
Python 3.7, Database postgres	57s
Python 3.8, Database sqlite	1m 30s
Python 3.8, Database postgres	1m 36s
Python 3.9, Database sqlite	1m 35s
Python 3.9, Database postgres	1m 34s
Python 3.10, Database sqlite	1m 59s
Python 3.10, Database postgres	2m 2s
Python 3.11, Database sqlite	1m 39s
Python 3.11, Database postgres	1m 36s
Python pypy-3.7, Database sqlite	1m 30s
Python pypy-3.7, Database postgres	1m 14s
Python pypy-3.8, Database sqlite	2m 27s
Python pypy-3.8, Database postgres	2m 12s
Python pypy-3.9, Database sqlite	2m 30s
Python pypy-3.9, Database postgres	2m 1s
	27m 28s

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