DOC: Unpin outdated RTD sphinx maxversion #283

Job	Run time
tests (Code style checks, ubuntu-latest, 3.x, codestyle)	32s
tests (ubuntu-latest, 3.8, py38-test-sphinx_oldest)	1m 16s
tests (ubuntu-latest, 3.8, py38-test-sphinx53)	1m 14s
tests (ubuntu-latest, 3.9, py39-test-sphinx62)	1m 11s
tests (ubuntu-latest, 3.9, py39-test-sphinx70)	2m 4s
tests (ubuntu-latest, 3.10, py310-test-sphinx71)	1m 7s
tests (ubuntu-latest, 3.11, py311-test-sphinx72-cov-clocale)	1m 17s
tests (ubuntu-latest, 3.12, py312-test-sphinx80)	1m 10s
tests (ubuntu-latest, 3.13, py313-test-sphinxdev)	1m 18s
tests (macos-latest, 3.10, py310-test-sphinx80-clocale)	2m 4s
tests (macos-latest, 3.13, py313-test-sphinxdev)	2m 0s
tests (windows-latest, 3.8, py38-test-sphinx_oldest)	2m 42s
tests (windows-latest, 3.10, py310-test-sphinx80)	2m 54s
tests (windows-latest, 3.13, py313-test-sphinxdev)	3m 7s
	23m 56s

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