Add support for cached_properties to slotted attrs classes. #731

Job	Run time
Build docs & run doctests	29s
Verify dev env (ubuntu-latest)	16s
Verify dev env (windows-latest)	47s
Check types using pyright	28s
Tests & Mypy on 3.7	47s
Tests & Mypy on 3.8	1m 9s
Tests & Mypy on 3.9	1m 8s
Tests & Mypy on 3.10	1m 12s
Tests & Mypy on 3.11	1m 19s
Tests & Mypy on 3.12	1m 24s
Tests & Mypy on pypy-3.8	1m 7s
Tests & Mypy on pypy-3.9	1m 2s
Tests & Mypy on pypy-3.10	1m 0s
Combine & check coverage.	18s
required-checks-pass	3s
	12m 29s

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