Can't track non-weak-referenceable objects

[epatters]

The object tracker, currently used in a crucial way to determine the flow graph topology, only works for objects that are weak referenceable. Types whose instances are not weak referenceable include:

• all builtin scalar types (int, float, bool, str, bytes, NoneType)
• some builtin container types (list, tuple, dict)
• a few miscallenous builtin types (slice)

This is a fairly serious defect.