descriptors for each attribute.  The getattr() implementation is
similar to PyObject_GenericGetAttr(), but delegates to im_self instead
of looking in __dict__; I couldn't do this as a wrapper around
PyObject_GenericGetAttr().

XXX A problem here is that this is a case of *delegation*.  dir()
doesn't see exactly the same attributes that are actually defined;
e.g. if the delegate is a Python function object, it supports
attributes like func_code etc., but these are not visible to dir(); on
the other hand, dynamic function attributes (stored in the function's
__dict__) *are* visible to dir().  Maybe we need a mechanism to tell
dir() about the delegation mechanism?  I vaguely recall seeing a
request in the newsgroup for a more formal definition of attribute
delegation too.  Sigh, time for a new PEP.