Crash because of ownership assert in `PyObject_ClearManagedDict`

[sobolevn]

Crash report

Refs #151763
Original description: https://gist.github.com/devdanzin/99b84915b197ae0ade7face262b8af66

Repro:

from unittest.mock import MagicMock
from _testcapi import set_nomemory

set_nomemory(200) # fail allocations from the 200th onward (lands in shutdown GC)
(MagicMock(), undefined_name) # build a MagicMock (managed __dict__), then NameError abandons
 # it into a traceback cycle -> survives to shutdown GC

I have a fix ready.

Linked PRs

• gh-152254: Fix a crash in PyObject_ClearManagedDict under memory pressure #152255