require a more recent version of `sphinx` #1107

Job	Run time
test-macos (3.9)	2m 27s
test-windows (3.9, numpy>=1.21,<2.0.0)	59s
test-macos (3.9, numpy>=1.21,<2.0.0)	4m 10s
test-linux (3.9)	1m 29s
test-windows (3.10, numpy>=1.21,<2.0.0)	57s
test-macos (3.10)	2m 53s
test-windows (3.11, numpy>=1.21,<2.0.0)	1m 5s
test-linux (3.9, uncertainties==3.1.6)	1m 38s
test-macos (3.10, numpy>=1.21,<2.0.0)	2m 58s
test-linux (3.9, uncertainties>=3.1.6,<4.0.0)	1m 36s
test-macos (3.11)	2m 28s
test-linux (3.9, numpy>=1.21,<2.0.0)	1m 35s
test-macos (3.11, numpy>=1.21,<2.0.0)	4m 2s
test-linux (3.9, numpy>=1.21,<2.0.0, uncertainties==3.1.6)	2m 8s
test-linux (3.9, numpy>=1.21,<2.0.0, uncertainties>=3.1.6,<4.0.0)	1m 57s
test-linux (3.10)	1m 39s
test-linux (3.10, uncertainties==3.1.6)	1m 44s
test-linux (3.10, uncertainties>=3.1.6,<4.0.0)	2m 13s
test-linux (3.10, numpy>=1.21,<2.0.0)	1m 45s
test-linux (3.10, numpy>=1.21,<2.0.0, uncertainties==3.1.6)	2m 13s
test-linux (3.10, numpy>=1.21,<2.0.0, uncertainties>=3.1.6,<4.0.0)	2m 5s
test-linux (3.11)	1m 36s
test-linux (3.11, uncertainties==3.1.6)	1m 35s
test-linux (3.11, uncertainties>=3.1.6,<4.0.0)	1m 53s
test-linux (3.11, numpy>=1.21,<2.0.0)	1m 39s
test-linux (3.11, numpy>=1.21,<2.0.0, uncertainties==3.1.6)	1m 53s
test-linux (3.11, numpy>=1.21,<2.0.0, uncertainties>=3.1.6,<4.0.0)	1m 54s
test-linux (3.9, numpy, matplotlib==2.2.5)	2m 38s
test-linux (3.9, numpy, uncertainties, sparse xarray netCDF4 dask[complete]==2023.4.0 graphviz ba...	2m 44s
	59m 53s

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