RFC: replace `#[pyclass(subclass)]` with `#[pyclass(final)]`

[davidhewitt]

Python types are subclassable by default, and use the @typing.final decorator to hint that they should not be subclassed.

We currently use #[pyclass(subclass)] to enable subclassing.

I wonder if we should instead move to #[pyclass(final)] to opt-out of subclassing, to match Python?

[alex]

While in general I think there's a good argument for this, I think in practice subclassing of native types is of pretty limited value. Since I don't think I've seen much feedback about this from users, I'd be inclined to leave well enough alone.

[mejrs]

I don't have a strong opinion either way, so I'd prefer to keep things as they are but if there's demand for it I also wouldn't be opposed.

[Icxolu]

Same for me, no strong opinion. If I had to pick I probably would slightly prefer the current design, but could also live with the other way around.