I'm working on a debugger, trying to identify instances of
old-style classes in Python 2, and any class in Python 3.
The getattr formulation will work, but because I already
need to maintain an "is_v3" flag, I might as well use it
here.

As a side note, how are instances of new-style classes in
v2 categorized?  This v2 code prints 'None':

import types
def get_type_name(target_type):
  for t in dir(types):
    if target_type == getattr(types, t):
      return t

class C(object):
  pass

c = C()
print get_type_name(type(c))