docs: use modern field API in 'boilerplate' section. #2082

Job	Run time
Build & verify package	17s
Verify dev env	9s
Check types using supported type checkers	21s
Tests on pypy-3.10	24s
Run doctests & render changelog	18s
Tests & Mypy on 3.10	1m 7s
Tests & Mypy on 3.12	1m 0s
Tests & Mypy on 3.14	1m 16s
Tests & Mypy on 3.11	58s
Tests & Mypy on 3.9	35s
Tests & Mypy on 3.13	1m 2s
Combine & check coverage	15s
required-checks-pass	3s
	7m 45s

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