Add support for cached_properties to slotted attrs classes. #715

Job	Run time
Build docs & run doctests	29s
Verify dev env (ubuntu-latest)	22s
Tests & Mypy on 3.7	23s
Verify dev env (windows-latest)	52s
Tests & Mypy on 3.8	1m 5s
Check types using pyright	33s
Tests & Mypy on 3.9	1m 4s
Tests & Mypy on 3.10	1m 46s
Tests & Mypy on 3.11	1m 58s
Tests & Mypy on 3.12	1m 11s
Tests & Mypy on pypy-3.8	1m 20s
Tests & Mypy on pypy-3.9	1m 16s
Tests & Mypy on pypy-3.10	1m 5s
Combine & check coverage.	0s
required-checks-pass	3s
	13m 27s

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