bpo-38912: regrtest logs unraisable exception into sys.__stderr__

[vstinner]

regrtest_unraisable_hook() temporarily replaces sys.stderr with
sys.stderr to help to display errors when a test captures stderr.

https://bugs.python.org/issue38912

[vstinner]

It should help to debug issues like https://bugs.python.org/issue38912

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Windows10 3.x has failed when building commit 701b638.

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/129/builds/1500) and take a look at the build logs.
4. Check if the failure is related to this commit (701b638) 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/129/builds/1500

Failed tests:

• test_tarfile

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

== Tests result: FAILURE then ENV CHANGED ==

392 tests OK.

10 slowest tests:

• test_mmap: 8 min 52 sec
• test_largefile: 6 min 19 sec
• test_io: 5 min 48 sec
• test_unparse: 3 min 49 sec
• test_tokenize: 2 min 53 sec
• test_multiprocessing_spawn: 2 min 21 sec
• test_concurrent_futures: 2 min 20 sec
• test_lib2to3: 2 min 3 sec
• test_peg_generator: 1 min 58 sec
• test_capi: 1 min 41 sec

1 test altered the execution environment:
test_asyncio

30 tests skipped:
test_curses test_dbm_gnu test_dbm_ndbm test_devpoll test_epoll
test_fcntl test_fork1 test_gdb test_grp test_ioctl test_kqueue
test_multiprocessing_fork test_multiprocessing_forkserver test_nis
test_openpty test_ossaudiodev test_pipes test_poll test_posix
test_pty test_pwd test_readline test_resource test_spwd
test_syslog test_threadsignals test_wait3 test_wait4
test_xxtestfuzz test_zipfile64

1 re-run test:
test_tarfile

Total duration: 20 min 15 sec

Click to see traceback logs

Traceback (most recent call last):
  File "D:\buildarea\3.x.bolen-windows10\build\lib\asyncio\proactor_events.py", line 115, in __del__
    _warn(f"unclosed transport {self!r}", ResourceWarning, source=self)
  File "D:\buildarea\3.x.bolen-windows10\build\lib\asyncio\proactor_events.py", line 79, in __repr__
    info.append(f'fd={self._sock.fileno()}')
  File "D:\buildarea\3.x.bolen-windows10\build\lib\asyncio\windows_utils.py", line 102, in fileno
    raise ValueError("I/O operation on closed pipe")
ValueError: I/O operation on closed pipe