test_asyncio.test_subprocess fails with "env changed" on "AMD64 Debian root 3.x" buildbot: unclosed file <_io.FileIO name=11 mode='wb' closefd=True>

[vstinner]

Time to time, test_asyncio.test_subprocess fails with "env changed" on "AMD64 Debian root 3.x" buildbot.

Example: https://buildbot.python.org/#/builders/2019/builds/114

0:15:58 load avg: 1.70 [450/505/1] test.test_asyncio.test_subprocess failed (env changed)
test_cancel_make_subprocess_transport_exec (test.test_asyncio.test_subprocess.SubprocessPidfdWatcherTests.test_cancel_make_subprocess_transport_exec) ... ok
(...)
test_subprocess_repr (test.test_asyncio.test_subprocess.SubprocessTransportTests.test_subprocess_repr) ... ok

----------------------------------------------------------------------
Ran 81 tests in 10.074s

OK
Warning -- Unraisable exception
Exception ignored while finalizing file <_io.FileIO name=11 mode='wb' closefd=True>:
Traceback (most recent call last):
  File "/root/buildarea/3.15.angelico-debian-amd64/build/Lib/test/support/__init__.py", line 948, in gc_collect
    gc.collect()
    ~~~~~~~~~~^^
ResourceWarning: unclosed file <_io.FileIO name=11 mode='wb' closefd=True>