There is no obvious way to deprecate a message schema

[abompard]

When a message schema must be changed in a backwards-incompatible way, a new class & schema must be written, but there is no obvious way nor helper to mark the old class as deprecated. It's always possible to raise a DeprecationWarning in the constructor, but the sender app will get that warning for the duration of the transition period, which is not ideal.

There could be a class attribute that the get_message() function would test for and raise a DeprecationWarning when true, it would only raise the warning when the message is read from the broker, not produced.

[abompard]

CC @Zlopez