The only remaining test failures are due to changed pickle sizes.  I've
discovered two reasons for that so far:

  1) Python 3 pickles 'native strings' as BINUNICODE, Python 2 pickles
     'native strings' as SHORT_BINSTRING, which is 3 bytes shorter.

  2) Python 3 and also the 'pickle' module Python 2 pickles tuples like
     (UserDefinedClass, ) with an extra BINPUT opcode at the end,
     compared to 'cPickle' in Python 2 when given the same tuple:

        Python 2.7.3 (default, Sep 26 2012, 21:51:14)
        [GCC 4.7.2] on linux2
        Type "help", "copyright", "credits" or "license" for more information.
        >>> import pickle, cPickle
        >>> class MyClass(object): pass
        ...
        >>> len(pickle.dumps((MyClass, None), 1))
        26
        >>> len(cPickle.dumps((MyClass, None), 1))
        24

The plan is to switch to zodbpickle instead of the builtin Python 3
pickle, then use encoding=bytes from http://bugs.python.org/issue6784
and fix the class-in-tuple encoding to match Python 2's cPickle.