Add helpers for deprecating classes.

Review Request #13013 — Created May 4, 2023 and submitted — Latest diff uploaded

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housekeeping
master

Reviewers

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.

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