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>

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>

Start with test/test_asyncio/test_subprocess.py and the test_asyncio.test_subprocess failure shown in the AMD64 Debian buildbot log. Run the relevant test and inspect how subprocess resources are finalized around the failing tests. Done means the intermittent "env changed" failure and unclosed FileIO ResourceWarning no longer occur.