chore: refactor json_io #407

Job	Run time
Tests on paywalled platforms	0s
Tox test (py38)	55s
Tests on other platforms (windows-2019, 3.10)	4m 36s
Tox test (py39)	3m 48s
Tests on other platforms (windows-2022, 3.11)	3m 45s
Tox test (py310)	3m 46s
Tests on other platforms (windows-2022, 3.12)	3m 56s
Tox test (py311)	3m 36s
Tests on other platforms (macos-13, 3.12)	4m 36s
Tox test (py312-cover)	6m 33s
Tests on other platforms (macos-14, 3.12)	2m 5s
Tox test (docs, graphviz)	1m 55s
Docker image	0s
PyPI packaging	0s
	39m 31s

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