gh-156466: fix cleanup on error in codegen_class_body - #156507

Thanks @iritkatriel for the PR 🌮🎉.. I'm working now to backport this PR to: 3.14, 3.15.
🐍🍒⛏🤖

GH-156535 is a backport of this pull request to the 3.15 branch.

GH-156536 is a backport of this pull request to the 3.14 branch.

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot s390x Fedora Stable Refleaks 3.14 (tier-3) has failed when building commit 28b944c.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/1776/builds/1423) and take a look at the build logs.
4. Check if the failure is related to this commit (28b944c) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/1776/builds/1423

Failed tests:

• test_external_inspection

Failed subtests:

• test_only_active_thread - test.test_external_inspection.TestGetStackTrace.test_only_active_thread

Summary of the results of the build (if available):

==

Traceback (most recent call last):
  File "/var/lib/buildbot/worker/cstratak-fedora-stable-s390x/3.14.cstratak-fedora-stable-s390x.refleak/build/Lib/test/test_external_inspection.py", line 1289, in test_only_active_thread
    self.assertEqual(
    ~~~~~~~~~~~~~~~~^
        len(gil_traces), 1, "Should have exactly one GIL holder"
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
AssertionError: 0 != 1 : Should have exactly one GIL holder