Simplify slot field detection #883

Job	Run time
Build docs & run doctests	16s
Verify dev env (ubuntu-latest)	21s
Check types using pyright	25s
Tests & Mypy on 3.7	52s
Verify dev env (windows-latest)	48s
Tests & Mypy on 3.8	50s
Tests & Mypy on 3.9	57s
Tests & Mypy on 3.10	1m 7s
Tests & Mypy on 3.11	54s
Tests & Mypy on 3.12	1m 8s
Tests & Mypy on 3.13	2m 3s
Tests & Mypy on pypy-3.8	51s
Tests & Mypy on pypy-3.9	45s
Tests & Mypy on pypy-3.10	46s
Combine & check coverage.	12s
required-checks-pass	1s
	12m 16s

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