fix docs requirements #223

Job	Run time
Tox test (py38)	1m 40s
Tests on paywalled platforms	0s
Tox test (py39)	1m 42s
Tests on other platforms (windows-2019, 3.10)	3m 12s
Tox test (py310)	1m 42s
Tests on other platforms (windows-2022, 3.11)	3m 27s
Tox test (py311)	1m 41s
Tests on other platforms (windows-2022, 3.12)	2m 36s
Tox test (py312-cover)	2m 40s
Tests on other platforms (macos-12, 3.11)	2m 49s
Tox test (docs, graphviz)	1m 48s
Tests on other platforms (macos-13, 3.12)	1m 50s
PyPI packaging	0s
Docker image	0s
	25m 7s

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