test_subprocess is failing on Windows.

C:\py\cpython3.7>./python -m test -uall -v -m test_nonexisting_with_pipes test_subprocess
Running Debug|Win32 interpreter...
== CPython 3.7.0b1+ (heads/3.7:1a0239e, Feb 4 2018, 16:19:37) [MSC v.1911 32 bit (Intel)]
== Windows-10-10.0.16299-SP0 little-endian
== cwd: C:\py\cpython3.7\build\test_python_11092
== CPU count: 2
== encodings: locale=cp1251, FS=utf-8
Run tests sequentially
0:00:00 [1/1] test_subprocess
test_nonexisting_with_pipes (test.test_subprocess.ProcessTestCase) ... FAIL
test_nonexisting_with_pipes (test.test_subprocess.ProcessTestCaseNoPoll) ... skipped 'Test needs selectors.PollSelector'

======================================================================
FAIL: test_nonexisting_with_pipes (test.test_subprocess.ProcessTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\py\cpython3.7\lib\test\test_subprocess.py", line 1194, in test_nonexisting_with_pipes
    self.assertEqual(stderr, "")
AssertionError: 'Traceback (most recent call last):\n  Fil[923 chars]le\n' != ''
Diff is 965 characters long. Set self.maxDiff to None to see it.

----------------------------------------------------------------------
Ran 2 tests in 0.171s

FAILED (failures=1, skipped=1)
test test_subprocess failed
test_subprocess failed

1 test failed:
    test_subprocess

Total duration: 391 ms
Tests result: FAILURE


Here stderr is:

Traceback (most recent call last):
  File "C:\py\cpython3.7\lib\subprocess.py", line 1101, in _execute_child
    args = os.fsdecode(args)  # os.PathLike -> str
  File "C:\py\cpython3.7\\lib\os.py", line 821, in fsdecode
    filename = fspath(filename)  # Does type-checking of `filename`.
TypeError: expected str, bytes or os.PathLike object, not list

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<string>", line 16, in <module>
  File "C:\py\cpython3.7\lib\subprocess.py", line 756, in __init__
    restore_signals, start_new_session)
  File "C:\py\cpython3.7\lib\subprocess.py", line 1104, in _execute_child
    args[0] = os.fsdecode(args[0])  # os.PathLike -> str
  File "C:\py\cpython3.7\\lib\os.py", line 821, in fsdecode
    filename = fspath(filename)  # Does type-checking of `filename`.
TypeError: expected str, bytes or os.PathLike object, not tuple

In _execute_child args is passed to os.fsdecode() unless it is a string. In this case args is a list. os.fsdecode() doesn't accept a list.

The regression was introduced in issue31961.