Better error message if `Generic.__init_subclass__` is not called

[JelleZijlstra]

Bug report

Bug description:

This code (from https://discuss.python.org/t/type-object-cls-has-no-attribute-parameters/104276/4 ):

class Base:
    def __init_subclass__(cls) -> None:
        pass

class Sub[T](Base): pass

class SubSub(Sub[object]): pass

Fails with an inscrutable error about cls.__parameters__ not existing, because super().__init_subclass__() was not called.

Let's produce a clearer error message in this case so it's easier to debug for the next person.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-139905: Provide suggestion in error message if Generic.__init_subclass__ was not called #139943
• gh-139905: better error msg when subclassing GenericAlias without calling __init_subclass__ #139949
• [3.14] gh-139905: Provide suggestion in error message if Generic.__init_subclass__ was not called (GH-139943) #139955
• [3.13] gh-139905: Provide suggestion in error message if Generic.__init_subclass__ was not called (GH-139943) #139956