docs: don't repeat top-level dependencies #180

Job	Run time
Tox test (py38)	2m 3s
Tests on other platforms (windows-2019, 3.10)	3m 27s
Tests on paywalled platforms (macos-13-xlarge, 3.12)	49s
Tox test (py39)	2m 1s
Tests on other platforms (windows-2022, 3.11)	3m 10s
Tox test (py310)	1m 58s
Tests on other platforms (windows-2022, 3.12)	2m 39s
Tox test (py311)	1m 55s
Tests on other platforms (macos-12, 3.11)	2m 49s
Tox test (py312-cover)	3m 6s
Tests on other platforms (macos-13, 3.12)	1m 47s
Tox test (docs, graphviz)	1m 32s
Docker image	0s
PyPI packaging	0s
	27m 16s

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