builtin_dir(): Treat classic classes like types. Use PyDict_Keys instead

builtin_dir():  Treat classic classes like types.  Use PyDict_Keys instead
of PyMapping_Keys because we know we have a real dict.  Tolerate that
objects may have an attr named "__dict__" that's not a dict (Py_None
popped up during testing).

test_descr.py, test_dir():  Test the new classic-class behavior; beef up
the new-style class test similarly.

test_pyclbr.py, checkModule():  dir(C) is no longer a synonym for
C.__dict__.keys() when C is a classic class (looks like the same thing
that burned distutils! -- should it be *made* a synoym again?  Then it
would be inconsistent with new-style class behavior.).