Parent does not have a cdef class default hash #3250
Description
Parent does has a default hash for python classes, but not for cython classes. This leads to very subtle bugs where converting a python class to a cython class can result in coercion failing mysteriously. Either coercion should be more descriptive with its error messages resulting from lack of a hash function or Parent should get a default cdef hash function.
CC: @robertwb
Component: coercion
Issue created by migration from https://trac.sagemath.org/ticket/3250