Convert CFieldObject tp_members to tp_getset, since there is no

Convert CFieldObject tp_members to tp_getset, since there is no
structmember typecode for Py_ssize_t fields.  This should fix some of
the errors on the PPC64 debian machine (64-bit, big endian).

Assigning to readonly fields now raises AttributeError instead of
TypeError, so the testcase has to be changed as well.