This introduces ClassDeprecatedMixin and ClassMovedMixin.
ClassDeprecatedMixin marks a class as deprecated, emitting a warning

when directly instantiated or subclassed. Warnings are not emitted when

instantiating or subclassing any subclasses of the class, though.
ClassMovedMixin is similar, but it communicates that a class has moved

elsewhere. It can either take the new class to point to as parameter, or

it can be mixxed into a class that subclasses the new class (useful when

a legacy base class needs to subclass a newer base class).
These make use of __init_subclass__ and keyword arguments in the

subclass list to handle storing state and notifying on subclass. This

gives us metaclass-like functionality without the subclass having to

define state in the class body.

Unit tests pass on all supported versions of Python.