More info with my latest change.

AMD64 Windows10 3.x:
https://buildbot.python.org/all/#/builders/129/builds/1500

0:10:26 load avg: 3.17 [354/423/2] test_asyncio failed (env changed) (1 min 23 sec) -- running: test_capi (1 min 3 sec)

Warning -- Unraisable exception
Exception ignored in: <function _ProactorBasePipeTransport.__del__ at 0x000001E0C873CCD0>
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


It seems like the issue comes from SubprocessProactorTests. Using python -m test.bisect_cmd, I isolated one test which causes the issue:

+ C:\vstinner\python\master\PCbuild\amd64\python_d.exe -m test --matchfile C:\Users\vstinner\AppData\Local\Temp\tmpoe9_zui_ --fail-env-changed test_asyncio -v
== CPython 3.10.0a0 (heads/master:701b63894f, Aug 3 2020, 23:38:07) [MSC v.1916 64 bit (AMD64)]        
== Windows-10-10.0.18362-SP0 little-endian
== cwd: C:\vstinner\python\master\build\test_python_7732æ
== CPU count: 2
== encodings: locale=cp1252, FS=utf-8
0:00:00 Run tests sequentially
0:00:00 [1/1] test_asyncio
test_cancel_post_init (test.test_asyncio.test_subprocess.SubprocessProactorTests) ... ok

----------------------------------------------------------------------

Ran 1 test in 0.015s

OK
C:\vstinner\python\master\lib\asyncio\windows_utils.py:112: ResourceWarning: unclosed <PipeHandle handle=292>
  _warn(f"unclosed {self!r}", ResourceWarning, source=self)
ResourceWarning: Enable tracemalloc to get the object allocation traceback
C:\vstinner\python\master\lib\asyncio\windows_utils.py:112: ResourceWarning: unclosed <PipeHandle handle=792>
  _warn(f"unclosed {self!r}", ResourceWarning, source=self)
ResourceWarning: Enable tracemalloc to get the object allocation traceback

Warning -- Unraisable exception
Exception ignored in: <function _ProactorBasePipeTransport.__del__ at 0x0000015D1FCA2AF0>
Traceback (most recent call last):
  File "C:\vstinner\python\master\lib\asyncio\proactor_events.py", line 115, in __del__
    _warn(f"unclosed transport {self!r}", ResourceWarning, source=self)
  File "C:\vstinner\python\master\lib\asyncio\proactor_events.py", line 79, in __repr__
    info.append(f'fd={self._sock.fileno()}')
  File "C:\vstinner\python\master\lib\asyncio\windows_utils.py", line 102, in fileno
    raise ValueError("I/O operation on closed pipe")
ValueError: I/O operation on closed pipe

Warning -- Unraisable exception
Exception ignored in: <function _ProactorBasePipeTransport.__del__ at 0x0000015D1FCA2AF0>
Traceback (most recent call last):
  File "C:\vstinner\python\master\lib\asyncio\proactor_events.py", line 115, in __del__
    _warn(f"unclosed transport {self!r}", ResourceWarning, source=self)
  File "C:\vstinner\python\master\lib\asyncio\proactor_events.py", line 79, in __repr__
    info.append(f'fd={self._sock.fileno()}')
  File "C:\vstinner\python\master\lib\asyncio\windows_utils.py", line 102, in fileno
    raise ValueError("I/O operation on closed pipe")
ValueError: I/O operation on closed pipe
test_asyncio failed (env changed)

== Tests result: ENV CHANGED ==