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