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IMO bool is a special case because it's meant to be a final class, and the
implementation of int (which is in C and so can violate most rules) doesn't
respect that.

But in general I think the only reasonable approach is that a construction
class method should return an instance of the subclass; these class methods
have a signature that's constrained by their signature in the base class.

OTOH operators like __add__ cannot be expected to return an instance of the
subclass, because these typically construct an instance using
__new__/__init__, whose signatures are *not* constrained by the base class.
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