docs: use the default theme on ReadTheDocs.org as well #193

Job	Run time
Tox test (py38)	1m 59s
Tests on paywalled platforms (macos-13-xlarge, 3.12)	49s
Tox test (py39)	2m 1s
Tests on other platforms (windows-2019, 3.10)	3m 17s
Tox test (py310)	2m 8s
Tests on other platforms (windows-2022, 3.11)	2m 50s
Tox test (py311)	2m 1s
Tests on other platforms (windows-2022, 3.12)	3m 0s
Tests on other platforms (macos-12, 3.11)	1m 53s
Tox test (py312-cover)	3m 0s
Tests on other platforms (macos-13, 3.12)	2m 22s
Tox test (docs, graphviz)	3m 50s
Docker image	1m 2s
PyPI packaging	0s
	30m 12s

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