Using `PyList` as a baseclass

[Anfid]

I'd like to subclass PyList, but it seems like PySizedLayout isn't implemented for it, so it can't be subclassed.

My use case is that I need to create a couple of custom list types with small bits of extra logic on each list update, but for now my alternative is to manually reimplement all the functionality of python lists, add that extra bit of logic on top and register the new class as a PySequence subclass. This results in large amounts of boilerplate.

I can already subclass PyDict and PySet, is there any reason that I'm missing why it's not allowed for PyList?

[mejrs]

PyList is dynamically sized, but we only support inheriting types that we can get the size of at compile time. There's an open PR for it, but there's been little progress on that.