doc: pin sphinx #3782

Job	Run time
arches	1s
mir-libs (amd64)	24m 50s
mir-libs (armhf)	44m 11s
mir-libs (arm64)	39m 53s
snap (MirServer/mir-test-tools, amd64)	24m 8s
snap (MirServer/mir-test-tools, armhf)	40m 9s
snap (MirServer/mir-test-tools, arm64)	29m 51s
snap (MirServer/confined-shell-wip, amd64)	23m 8s
snap (MirServer/confined-shell-wip, armhf)	29m 49s
snap (MirServer/confined-shell-wip, arm64)	31m 14s
snap (MirServer/ubuntu-frame, amd64)	19m 59s
snap (MirServer/ubuntu-frame, armhf)	23m 12s
snap (MirServer/ubuntu-frame, arm64)	19m 32s
snap (Miriway/Miriway, amd64)	20m 42s
snap (Miriway/Miriway, armhf)	28m 39s
snap (Miriway/Miriway, arm64)	31m 34s
	7h 10m 52s

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