Fix slotted reference cycles on 3.14 #1593

Job	Run time
Verify dev env	13s
Check types using pyright	20s
Build & verify package	13s
Tests on pypy-3.10	18s
Tests & Mypy on 3.13	1m 5s
Tests & Mypy on 3.9	33s
Run doctests & render changelog	14s
Tests & Mypy on 3.10	1m 17s
Tests & Mypy on 3.14	2m 53s
Tests & Mypy on 3.11	58s
Tests & Mypy on 3.12	1m 8s
Combine & check coverage	10s
required-checks-pass	4s
	9m 26s

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