Static attribute that is True only for a concrete class, and False for its children in Python

The Problem

Say I have a class Root and want to access (e.g. initialise) all its subclasses. But there can be some subclasses that need to be ignored programatically.

Example

class Root(object):
    pass


class Parent(Root):
    ignore_me = True


class Child(Parent):
    pass


subs = [sub for sub in Root.__subclasses__() if not sub.ignore_me]

So what I want here is that Child class is included in the subs list as opposed to the Parent class.

Trivial Solution

Of course, I could define the ignore_me attribute for each subclass, but the point is that I want to isolate the subclasses from that detail, so that they will not even be aware of it.

Question

How can I achieve the goal by just defining the ignore_me attribute only in Parent class?