Prepare release 4.0.0

Run `towncrier build` for 4.0.0 and then curate the result by hand: the
release opens with a note that 4.0 requires Python 3.11 and drops `six`,
the `isoparse` entry is first among the bugfixes because it rejects
strings that used to parse, the 1051, 1583 and 1625 entries are
reworded, and the Hatchling and pyproject.toml entries are combined. The
remaining CI-only entries in "Misc" are collapsed into a list of PR
numbers.