bpo-42639: atexit._run_exitfuncs() uses sys.unraisablehook

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atexit._run_exitfuncs() now logs callback exceptions using
sys.unraisablehook, rather than logging them directly into
sys.stderr and raise the last exception.

Run GeneralTest of test_atexit in a subprocess since it calls
atexit._clear() which clears all atexit callbacks.

_PyAtExit_Fini() sets state->callbacks to NULL.

https://bugs.python.org/issue42639

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This PR also fix a test_atexit bug: it had a side effect, clearing all atexit callbacks.

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cc @encukou

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot s390x RHEL8 LTO 3.x has failed when building commit 3ca2b8f.

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/all/#builders/567/builds/1147) and take a look at the build logs.
4. Check if the failure is related to this commit (3ca2b8f) 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/all/#builders/567/builds/1147

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

== Tests result: ENV CHANGED ==

411 tests OK.

10 slowest tests:

• test_concurrent_futures: 2 min 58 sec
• test_peg_generator: 2 min 46 sec
• test_gdb: 1 min 36 sec
• test_multiprocessing_spawn: 1 min 10 sec
• test_multiprocessing_forkserver: 1 min 5 sec
• test_multiprocessing_fork: 54.3 sec
• test_asyncio: 49.3 sec
• test_signal: 47.1 sec
• test_tokenize: 34.2 sec
• test_io: 33.6 sec

1 test altered the execution environment:
test_asyncio

14 tests skipped:
test_devpoll test_ioctl test_kqueue test_msilib test_nis
test_ossaudiodev test_startfile test_tix test_tk test_ttk_guionly
test_winconsoleio test_winreg test_winsound test_zipfile64

Total duration: 4 min 53 sec

Click to see traceback logs

Traceback (most recent call last):
  File "/home/dje/cpython-buildarea/3.x.edelsohn-rhel8-z.lto/build/Lib/asyncio/sslproto.py", line 321, in __del__
    self.close()
  File "/home/dje/cpython-buildarea/3.x.edelsohn-rhel8-z.lto/build/Lib/asyncio/sslproto.py", line 316, in close
    self._ssl_protocol._start_shutdown()
  File "/home/dje/cpython-buildarea/3.x.edelsohn-rhel8-z.lto/build/Lib/asyncio/sslproto.py", line 590, in _start_shutdown
    self._abort()
  File "/home/dje/cpython-buildarea/3.x.edelsohn-rhel8-z.lto/build/Lib/asyncio/sslproto.py", line 731, in _abort
    self._transport.abort()
  File "/home/dje/cpython-buildarea/3.x.edelsohn-rhel8-z.lto/build/Lib/asyncio/selector_events.py", line 680, in abort
    self._force_close(None)
  File "/home/dje/cpython-buildarea/3.x.edelsohn-rhel8-z.lto/build/Lib/asyncio/selector_events.py", line 731, in _force_close
    self._loop.call_soon(self._call_connection_lost, exc)
  File "/home/dje/cpython-buildarea/3.x.edelsohn-rhel8-z.lto/build/Lib/asyncio/base_events.py", line 745, in call_soon
    self._check_closed()
  File "/home/dje/cpython-buildarea/3.x.edelsohn-rhel8-z.lto/build/Lib/asyncio/base_events.py", line 510, in _check_closed
    raise RuntimeError('Event loop is closed')
RuntimeError: Event loop is closed

[encukou]

Nice! Thanks for simplifying this!