Add support for cached_properties to slotted attrs classes. #718

Job	Run time
Build docs & run doctests	24s
Verify dev env (ubuntu-latest)	25s
Check types using pyright	25s
Verify dev env (windows-latest)	48s
Tests & Mypy on 3.7	40s
Tests & Mypy on 3.8	1m 7s
Tests & Mypy on 3.9	59s
Tests & Mypy on 3.10	1m 18s
Tests & Mypy on 3.11	1m 22s
Tests & Mypy on 3.12	1m 15s
Tests & Mypy on pypy-3.8	58s
Tests & Mypy on pypy-3.9	58s
Tests & Mypy on pypy-3.10	59s
Combine & check coverage.	10s
required-checks-pass	1s
	11m 49s

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