refactor(spec): Replace `@slotted_freezable` with `@dataclasses(frozen=True,slots=True)`

[petertdavies]

The functionality that @slotted_freezable implements has been available out of the box in python dataclasses since Python 3.10.

Uses of modify() will need to be replaced by dataclasses.replace(). This should be straightforward refactor.

[SamWilsn]

There was a reason we didn't use frozen straight up. I can't remember what it was, but there was one.

[petertdavies]

There was a reason we didn't use frozen straight up. I can't remember what it was, but there was one.

IIRC, the problem was that slots didn't exist at the time. I implemented @slotted_freezable on Jul 29, 2021, slots support was added to Python 3.10 (released Oct 4th, 2021).