fail-on-template-vars: improve compatibility with Django behavior #487

Job	Run time
test (linting,docs, 3.12, false)	33s
test (py312-dj51-postgres-xdist-coverage, 3.12, false)	2m 56s
test (py312-dj42-postgres-xdist-coverage, 3.12, false)	3m 10s
test (py311-dj50-postgres-xdist-coverage, 3.11, false)	1m 57s
test (py311-dj42-postgres-xdist-coverage, 3.11, false)	2m 6s
test (py310-dj51-postgres-xdist-coverage, 3.10, false)	2m 5s
test (py310-dj42-postgres-xdist-coverage, 3.10, false)	2m 2s
test (py311-dj51-mysql_innodb-coverage, 3.11, false)	2m 19s
test (py310-dj42-mysql_innodb-coverage, 3.10, false)	1m 54s
test (py39-dj42-mysql_innodb-xdist-coverage, 3.9, false)	2m 16s
test (py312-dj51-sqlite-xdist-coverage, 3.12, false)	2m 20s
test (py38-dj42-sqlite-xdist-coverage, 3.8, false)	1m 34s
test (py311-djmain-sqlite-coverage, 3.11, true)	1m 42s
test (py311-dj42-mysql_myisam-coverage, 3.11, false)	1m 49s
test (py38-dj42-sqlite-pytestmin-coverage, 3.8, false)	1m 32s
test (pypy3-dj42-postgres, pypy3.9, false)	2m 44s
	32m 59s

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