requirements: update matplotlib requirement from <3.7.6 to <3.9.4 #5681

Job	Run time
build (3.7, x64, ubuntu)	55m 20s
build (3.7, x64, windows)	52m 8s
build (3.11, x64, ubuntu)	1h 24m 1s
build (3.11, x64, macos)	25m 8s
build (3.11, x64, windows)	1h 25m 6s
build (3.12, x64, ubuntu)	1h 20m 14s
build (3.12, x64, macos)	31m 26s
build (3.12, x64, windows)	1h 22m 10s
build (3.9, macos, --cov=psyneulink)	56m 29s
build (3.9, ubuntu, --forked -m "not llvm")	33m 5s
build (3.8, x86, windows)	1h 31m 18s
build (3.10, ubuntu, --fp-precision=fp32)	1h 22m 15s
build (3.10, macos, -m benchmark --benchmark-enable --benchmark-only --benchmark-min-rounds=2 --b...	29m 26s
build (3.7, x64, ubuntu, min)	54m 22s
build (3.8, macos)	36m 17s
	14h 38m 45s

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