[3.14] pythongh-137568: Ignore startup file in test_dumb_terminal_exits_cleanly (pythonGH-140295) (pythonGH-140628)

pythongh-137568: Ignore startup file in `test_dumb_terminal_exits_cleanly` (pythonGH-140295)
(cherry picked from commit 06c7794)

Co-authored-by: Bartosz Sławecki <[email protected]>

Author: miss-islington

Lib/test/test_pyrepl/test_pyrepl.py

@@ -1406,6 +1406,9 @@ class TestDumbTerminal(ReplTestCase):
     def test_dumb_terminal_exits_cleanly(self):
         env = os.environ.copy()
         env.pop('PYTHON_BASIC_REPL', None)
+        # Ignore PYTHONSTARTUP to not pollute the output
+        # with an unrelated traceback. See GH-137568.
+        env.pop('PYTHONSTARTUP', None)
         env.update({"TERM": "dumb"})
         output, exit_code = self.run_repl("exit()\n", env=env)
         self.assertEqual(exit_code, 0)