Refactor: define a separate function to compute targets #230

Job	Run time
Tests on paywalled platforms (macos-13-xlarge, 3.12)	1m 9s
Tests on other platforms (windows-2019, 3.10)	3m 21s
Tox test (py38)	2m 43s
Tests on other platforms (windows-2022, 3.11)	3m 22s
Tox test (py39)	2m 57s
Tests on other platforms (windows-2022, 3.12)	3m 43s
Tox test (py310)	2m 49s
Tests on other platforms (macos-12, 3.11)	2m 28s
Tox test (py311)	2m 34s
Tests on other platforms (macos-13, 3.12)	3m 25s
Tox test (py312-cover)	5m 18s
Tox test (docs, graphviz)	1m 55s
Docker image	58s
PyPI packaging	0s
	36m 42s

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