Fix breakage with zodbpickle >= 2
Starting from zodbpickle 2 its binary class does not allow users to set arbitrary attributes and so
binary._pack = bytes.__str__fails with
TypeError: can't set attributes of built-in/extension type 'zodbpickle.binary'-> Fix it by explicitly checking for binary type on encoding instead of setting binary._pack
See slapos@27f574bc for pre-history.
/cc @jerome