docs: try to unbreak the readthedocs.io build #174

Job	Run time
Tests on paywalled platforms (macos-13-xlarge, 3.12)	50s
Tests on other platforms (windows-2019, 3.10)	2m 49s
Tox test (py38)	2m 6s
Tests on other platforms (windows-2022, 3.11)	2m 40s
Tox test (py39)	2m 4s
Tests on other platforms (windows-2022, 3.12)	2m 45s
Tox test (py310)	2m 5s
Tests on other platforms (macos-12, 3.11)	1m 53s
Tox test (py311)	2m 5s
Tests on other platforms (macos-13, 3.12)	2m 58s
Tox test (py312-cover)	3m 10s
Tox test (docs, graphviz)	1m 58s
Docker image	0s
PyPI packaging	0s
	27m 23s

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