No "post-init only" marker about dataclassy

[biqqles]

No, it's not already possible. I originally didn't want to implement something like late_init (taking the name from Kotlin) because I think it means checking the type before generating __init__ which we don't do for anything else, so it would be messy. Even factory is implemented without needing to do this. I also thought it was neat just to be able to simply assign attributes in __post_init__. That said it never occurred to me that missing it out would be a problem for slotted classes.

I need to think about this more to work out how best to implement the feature.