docs: use modern field API in 'boilerplate' section. #2084

Job	Run time
Check types using supported type checkers	21s
Verify dev env	8s
Build & verify package	17s
Run doctests & render changelog	15s
Tests on pypy-3.10	25s
Tests & Mypy on 3.12	1m 6s
Tests & Mypy on 3.9	34s
Tests & Mypy on 3.13	1m 4s
Tests & Mypy on 3.14	1m 9s
Tests & Mypy on 3.10	1m 9s
Tests & Mypy on 3.11	58s
Combine & check coverage	8s
required-checks-pass	6s
	7m 40s

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