mg composition as weakref #1570

Job	Run time
select-runner	1s
build (3.7, x64, ubuntu)	1h 8m 58s
build (3.7, x64, windows)	1h 14m 2s
build (3.11, x64, ubuntu)	1h 42m 38s
build (3.11, x64, macos)	1h 21m 56s
build (3.11, x64, windows)	1h 59m 41s
build (3.9, macos, --cov=psyneulink)	1h 56m 8s
build (3.9, ubuntu, --forked -m "not llvm")	47m 15s
build (3.8, x86, windows)	1h 52m 45s
build (3.10, ubuntu, --fp-precision=fp32)	1h 35m 16s
build (3.10, macos, -m benchmark --benchmark-enable --benchmark-only --benchmark-min-rounds=2 --b...	49m 38s
build (3.7, x64, ubuntu, min)	1h 11m 3s
build (3.8, x64, macos)	1h 2m 7s
	16h 41m 28s

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