deserialize on Attribute subclasses should be a classmethod

[garrettheel]

I'll likely implement this in the future, just wanted to capture it here so I don't forget.

deserialize on attributes should be a class method rather than an instance method. Being an instance method means we awkwardly have to instantiate and throw away a new instance even though we never rely on anything in self (except for ListAttribute, which we'll need to think about)