Bump minimum sphinx version. #231

Job	Run time
a11y-tests (ubuntu-latest, 3.12)	3m 30s
build PST docs (ubuntu-latest, 3.12)	1m 17s
run-pytest (ubuntu-latest, 3.9)	1m 33s
build PST docs (macos-latest, 3.12)	2m 38s
run-pytest (ubuntu-latest, 3.10)	1m 36s
build PST docs (windows-latest, 3.12)	4m 48s
build PST docs (ubuntu-latest, 3.9, 6.1)	1m 31s
run-pytest (ubuntu-latest, 3.11)	1m 40s
run-pytest (ubuntu-latest, 3.12)	1m 56s
run-pytest (ubuntu-24.04, 3.11)	1m 26s
run-pytest (ubuntu-24.04, 3.12)	1m 44s
run-pytest (macos-14, 3.10)	2m 17s
run-pytest (macos-14, 3.11)	2m 9s
run-pytest (macos-14, 3.12)	1m 55s
run-pytest (macos-13, 3.9)	4m 14s
run-pytest (macos-13, 3.10)	3m 7s
run-pytest (macos-13, 3.11)	3m 59s
run-pytest (macos-13, 3.12)	5m 42s
run-pytest (windows-latest, 3.9)	3m 9s
run-pytest (windows-latest, 3.10)	2m 47s
run-pytest (windows-latest, 3.11)	2m 54s
run-pytest (windows-latest, 3.12)	3m 40s
run-pytest (ubuntu-latest, 3.9, 6.1)	1m 30s
run-pytest (ubuntu-latest, 3.12, dev)	1m 54s
lighthouse-audit	4m 20s
check coverage	34s
profiling	37s
	1h 8m 27s

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