fixtures: drop compat code for no longer supported Django versions #491

Job	Run time
test (linting,docs, 3.12, false)	31s
test (py312-dj51-postgres-xdist-coverage, 3.12, false)	3m 4s
test (py312-dj42-postgres-xdist-coverage, 3.12, false)	3m 1s
test (py311-dj50-postgres-xdist-coverage, 3.11, false)	2m 5s
test (py311-dj42-postgres-xdist-coverage, 3.11, false)	1m 54s
test (py310-dj51-postgres-xdist-coverage, 3.10, false)	2m 5s
test (py310-dj42-postgres-xdist-coverage, 3.10, false)	2m 3s
test (py311-dj51-mysql_innodb-coverage, 3.11, false)	2m 4s
test (py310-dj42-mysql_innodb-coverage, 3.10, false)	2m 13s
test (py39-dj42-mysql_innodb-xdist-coverage, 3.9, false)	2m 31s
test (py312-dj51-sqlite-xdist-coverage, 3.12, false)	2m 27s
test (py38-dj42-sqlite-xdist-coverage, 3.8, false)	1m 36s
test (py311-djmain-sqlite-coverage, 3.11, true)	1m 34s
test (py311-dj42-mysql_myisam-coverage, 3.11, false)	2m 3s
test (py38-dj42-sqlite-pytestmin-coverage, 3.8, false)	1m 32s
test (pypy3-dj42-postgres, pypy3.9, false)	2m 42s
	33m 25s

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