We have __slots__ with other ABC's, see http://bugs.python.org/issue11333 and http://bugs.python.org/issue21421.

There are no downsides to having empty slots on a non-instantiable class, but it does give the option of denying __dict__ creation for subclassers. 

The possibility of breaking is for someone using __slots__ but relying on __dict__ creation in a subclass - they will have to explicitly add "__dict__" to __slots__. Since we have added __slots__ to other ABC's, 

I will provide a PR soon on this. Diff should look like this (in Lib/abc.py):

class ABC(metaclass=ABCMeta):
    """Helper class that provides a standard way to create an ABC using
    inheritance.
    """
- pass
+ __slots__ = ()

(I also want to add a test for this, and ensure other ABC's also have this if they don't.)

This seems reasonable.

New changeset ff48739ed0a3f366c4d56d3c86a37cbdeec600de by Serhiy Storchaka (Aaron Hall, MBA) in branch 'master':
bpo-30463: Add an empty __slots__ to abc.ABC.
https://github.com/python/cpython/commit/ff48739ed0a3f366c4d56d3c86a37cbdeec600de

I will note that this means with:

class BaseClass(ABC):
    pass


class MyDerivedClass(BaseClass):

    def __init__(self, thing):
        self.thing = thing

thing = MyDerivedClass()

thing now has both __slots__ and, evidently, a dict. That's a bit weird and confusing.