`test_cmd_line.test_non_interactive_output_buffering` fails when `PYTHONUNBUFFERED` is enabled

[WolframAlph]

Bug report

Bug description:

Reproducer:

➜  cpython git:(main) ✗ export PYTHONUNBUFFERED=1         
➜  cpython git:(main) ✗ python3 -m test test_cmd_line
Using random seed: 2481773783
Raised RLIMIT_NOFILE: 256 -> 1024
0:00:00 load avg: 2.40 Run 1 test sequentially in a single process
0:00:00 load avg: 2.40 [1/1] test_cmd_line
test test_cmd_line failed -- Traceback (most recent call last):
  File "/opt/homebrew/Cellar/python@3.13/3.13.1/Frameworks/Python.framework/Versions/3.13/lib/python3.13/test/test_cmd_line.py", line 348, in test_non_interactive_output_buffering
    self.assertEqual(proc.stdout,
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^
                     'False False False\n'
                     ^^^^^^^^^^^^^^^^^^^^^
                     'False False True\n')
                     ^^^^^^^^^^^^^^^^^^^^^
AssertionError: 'False True False\nFalse True False\n' != 'False False False\nFalse False True\n'
- False True False
?       ^^^
+ False False False
?       ^^^^
- False True False
+ False False True


test_cmd_line failed (1 failure)

== Tests result: FAILURE ==

1 test failed:
    test_cmd_line

Total duration: 2.8 sec
Total tests: run=59 failures=1 skipped=1
Total test files: run=1/1 failed=1
Result: FAILURE

Test expects buffered stdio but does not check for PYTHONUNBUFFERED.
Init config for buffering:

cpython/Python/initconfig.c

Lines 1786 to 1790 in e389d6c

	int unbuffered_stdio = 0;
	_Py_get_env_flag(use_env, &unbuffered_stdio, "PYTHONUNBUFFERED");
	if (unbuffered_stdio) {
	config->buffered_stdio = 0;
	}

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-128377: Fix test_cmd_line.test_non_interactive_output_buffering fail with PYTHONUNBUFFERED=1 #128378

[hugovk]

Triage: I think we can close this now? Please comment on the PR if a backport is needed: #128378 (comment). Thanks!