docs: use modern field API in 'boilerplate' section. #2085

Job	Run time
Build & verify package	18s
Verify dev env	8s
Check types using supported type checkers	19s
Tests on pypy-3.10	25s
Run doctests & render changelog	16s
Tests & Mypy on 3.9	33s
Tests & Mypy on 3.12	1m 10s
Tests & Mypy on 3.10	1m 10s
Tests & Mypy on 3.14	1m 4s
Tests & Mypy on 3.11	54s
Tests & Mypy on 3.13	1m 3s
Combine & check coverage	12s
required-checks-pass	4s
	7m 36s

Provide feedback