Update sphinx to 8.1.2 #1790

Job	Run time
Build and test (ubuntu-latest, 3.8)	16s
Build and test (ubuntu-latest, 3.9)	19s
Build and test (ubuntu-latest, 3.10)	27s
Build and test (ubuntu-latest, 3.11)	23s
Build and test (ubuntu-latest, 3.12)	24s
Build and test (ubuntu-latest, 3.13-dev)	1m 25s
Build and test (windows-latest, 3.8)	1m 6s
Build and test (windows-latest, 3.9)	39s
Build and test (windows-latest, 3.10)	53s
Build and test (windows-latest, 3.11)	48s
Build and test (windows-latest, 3.12)	1m 33s
Build and test (windows-latest, 3.13-dev)	2m 0s
Build and test (macos-latest, 3.8)	43s
Build and test (macos-latest, 3.9)	18s
Build and test (macos-latest, 3.10)	17s
Build and test (macos-latest, 3.11)	21s
Build and test (macos-latest, 3.12)	22s
Build and test (macos-latest, 3.13-dev)	36s
	12m 50s

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