I found indeed 2 problems in regrtest.py.
I don't have svn access for the moment, so I cannot provide a regular
patch, so here are the changes:

- The line
    obj._abc_registry = abcs.get(obj, {}).copy()
incorrectly puts an empty dictionary in _abc_registry. It should be a set:
    from weakref import WeakSet
    obj._abc_registry = abcs.get(obj, WeakSet()).copy()

- This test in dash_R()
    if not isinstance(abc, _Abstract):
is not correct. It should be 
    if not issubclass(abc, _Abstract):
(Note that dash_R_cleanup contains the same line, but correctly)