ctypes.Structure and Iterable

Bug Report

For some reason ctypes.Structure instances pass mypy's isinstance checks although mypy also claims that they are not iterable.
For example, consider the following code:

from ctypes import Structure

from typing import Iterable


class S(Structure):
    pass


s = S()

if isinstance(s, Iterable):
    for _ in s:
        pass

mypy gives the following error:

test.py:13: error: "S" has no attribute "__iter__" (not iterable)
Found 1 error in 1 file (checked 1 source file)

replacing it with iter(s) make mypy happy again.

Your Environment

• Mypy version used: mypy 0.790
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: Python 3.8.6
• Operating system and version: Linux ubuntu 5.10.0 #17 SMP Mon Dec 14 10:04:17 IST 2020 x86_64 x86_64 x86_64 GNU/Linux

Start by running the reported Python reproducer with mypy and compare the isinstance(s, Iterable) branch with the iter(s) call. Trace the relevant type-checking behavior for ctypes.Structure and Iterable, then add a regression test showing the expected consistency.