Detect non-subclassable class being subclassed at compile-time

[ChayimFriedman2]

The following compiles, but crashes at runtime:

#[pyclass] // No `(subclass)`!
pub struct BaseClass {}

#[pyclass(extends=BaseClass)]
pub struct SubClass {}

It'd be nice if we could detect this at compile-time instead. This should be simple: make a bound that isn't true for non-subclassable classes, possibly even with a nice #[diagnostics::on_unimplemented].

I can work on this later.

[davidhewitt]

A thought to consider here, should we invert this and have final instead of subclass? Imo most python users expect to be able to subclass things...