semanal: Treat __init_subclass__ as an implicit classmethod

[tavianator]

Fixes #7352

[tavianator]

From my reading of https://www.python.org/dev/peps/pep-0487/, we should treat __init_subclass__ as if it were decorated with @classmethod, so this PR does that. Not sure the code is in the right place.

[ilevkivskyi]

Thanks for quick response!

TBH I don't understand completely how this fixes the issue (and how your first PR caused it), but I like the code I see here. It makes things more consistent. In addition I would update the comment for is_class slot in mypy/nodes.py. Currently it reads

'is_class',        # Uses "@classmethod"

I think we should make it

'is_class',        # Uses "@classmethod" (explicit or implicit)

so that people will not make false assertions about defining node. Could you please do this and then I will merge this.