Fix calling `delete` on instances using nested `ModelMocker` context managers #97

Job	Run time
Python 3.8 with packages py38-dj22-drf313	28s
Python 3.9 with packages py39-dj22-drf313	34s
Python 3.8 with packages py38-dj32-drf314	30s
Python 3.9 with packages py39-dj32-drf314	38s
Python 3.10 with packages py310-dj32-drf314	30s
Python 3.8 with packages py38-dj42-drf314	28s
Python 3.9 with packages py39-dj42-drf314	35s
Python 3.10 with packages py310-dj42-drf314	28s
Python 3.11 with packages py311-dj42-drf314	30s
Python 3.12 with packages py312-dj42-drf314	40s
Python 3.10 with packages py310-dj50-drf314	33s
Python 3.11 with packages py311-dj50-drf314	27s
Python 3.12 with packages py312-dj50-drf314	41s
	7m 2s

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