ctypes Stucture._fields_ cannot be set when Structure has been subclassed

[ionite34]

Bug report

Currently the method for defining Structure fields that cannot be defined within the class, is to define the class with pass and then set _fields_ outside. But ctypes.Structure incorrectly raises AttributeError, even when no class has ever set a _fields_.

from ctypes import *

class PyObject(Structure):
    pass

class PyVarObject(PyObject):
    pass

class PyTypeObject(PyVarObject):
    pass

PyObject._fields_ = [
    ("ob_refcnt", c_long),
    ("ob_type", POINTER(PyTypeObject)),
]

    PyObject._fields_ = [
    ^^^^^^^^^^^^^^^^^
AttributeError: _fields_ is final

Strangely, suppressing this Attribute error reveals the _fields_ attribute does actually get set?

with suppress(AttributeError):
    PyObject._fields_ = [
        ("ob_refcnt", c_long),
        ("ob_type", POINTER(PyTypeObject)),
    ]

print(PyObject._fields_)
>> [('ob_refcnt', <class 'ctypes.c_long'>), ('ob_type', <class '__main__.LP_PyTypeObject'>)]

Your environment

• CPython versions tested on: 3.11.1, main
• Operating system and architecture: macOS ARM, ubuntu x64