jupyter: erp5_kernel now correctly persists new-style classes without `_p_jar`

If ERP5 Jupyter kernel tried to persist an object created from a new-style class, it would crash because the object has no `_p_jar`. This makes the creation of the ObjectWriter fails. This kind of object needs to be handled by (c)Pickle.

Related to bug report https://nexedi.erp5.net/bug_module/20160819-882FA2, created by @kasra.

@Tyagov, please review when possible. 

/reviewed-on nexedi/erp5!162