Python 3.10 is the minimum, so annotations use `X | None`, `list[X]`
and friends, enforced by ruff's pyupgrade rules. The remaining typing
names are imported directly, with abstract types coming from
collections.abc. first() is now generic over the item and default.
ruff format keeps black's style at the same line length, and ruff
check covers import sorting plus pycodestyle and pyflakes. Three
comments and a test regex are wrapped to fit 79 columns.
Dash