What is __init_subclass__ in Python?

Question presented to candidate: "Have you come across __init_subclass__? What does it do, when exactly does it run, and when would you use it instead of a metaclass or a class decorator?"

What a strong answer should cover:

• __init_subclass__ is a method on a base class that Python calls every time a subclass of it is defined (at class statement time, not when instances are created), with cls bound to the new subclass; it is not called for the class that defines it.
• It is an implicit classmethod and was added in Python 3.6 by PEP 487 together with __set_name__.
• Class keyword arguments (class CsvExporter(Exporter, fmt="csv")) are passed to it; it should accept **kwargs and call super().__init_subclass__(**kwargs), and object.__init_subclass__ raises TypeError for leftover keywords.
• Typical uses: plugin registries, enforcing that subclasses define required attributes or methods, configuring subclasses — validation happens at import time.
• Order during class creation: class body, then __set_name__ on descriptors, then __init_subclass__; compared with a metaclass it composes through ordinary inheritance and avoids metaclass conflicts.

Code / implementation expected: Yes — a base class whose __init_subclass__ registers subclasses by a class keyword and rejects incomplete ones.