requirements: update sphinx requirement from <4.2.1 to <8.1.4 #5464

Job	Run time
build (3.7, x64, ubuntu)	49m 23s
build (3.7, x64, windows)	47m 2s
build (3.11, x64, ubuntu)	1h 13m 32s
build (3.11, x64, macos)	33m 44s
build (3.11, x64, windows)	1h 16m 38s
build (3.12, x64, ubuntu)	1h 14m 50s
build (3.12, x64, macos)	22m 3s
build (3.12, x64, windows)	1h 15m 3s
build (3.9, macos, --cov=psyneulink)	41m 6s
build (3.9, ubuntu, --forked -m "not llvm")	32m 39s
build (3.8, x86, windows)	1h 20m 6s
build (3.10, ubuntu, --fp-precision=fp32)	1h 14m 27s
build (3.10, macos, -m benchmark --benchmark-enable --benchmark-only --benchmark-min-rounds=2 --b...	21m 27s
build (3.7, x64, ubuntu, min)	49m 1s
build (3.8, macos)	23m 45s
	12h 54m 46s

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