For the Fractions issue, consider adding an empty __slots__ declaration
to every level in numbers.py.  That will preclude unintended dictionary
creation for anything inheriting from the numbers abcs.

We should also be an issue for the other abcs.  The collections ABCs are
less affected because they are not as granular as number objects.  So
their overall memory footprint only grows by a small percentage from the
inclusion of an instance dictionary.