Trait or subclass to manage deprecations

[vreuter]

To facilitate accommodation for backward compatibility, there could be a subclass or trait for declaring deprecated keys/attributes and their newer versions, with an inferred default message that could be overridden. This comes up a decent amount within pepkit and would probably be more broadly useful, facilitating backcompat by abstracting away some of the handling of the deprecation thus lowering the cost of providing it.

[nsheff]

interesting thought. That does seem useful.