gh-125470: Fix warning in Python/generated_cases.c.h

[sobolevn]

Refs #125324

• Issue: New warning: unused variable left_o[-Wunused-variable] in Python/generated_cases.c.h:186 #125470

[sobolevn]

Failures do not look related:

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FAIL: test_timerfd_poll (test.test_os.TimerfdTests.test_timerfd_poll)
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Traceback (most recent call last):
  File "/home/runner/work/cpython/cpython/Lib/test/test_os.py", line 4391, in test_timerfd_poll
    self.check_timerfd_poll(False)
    ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^
  File "/home/runner/work/cpython/cpython/Lib/test/test_os.py", line 4387, in check_timerfd_poll
    self.assertGreater(dt, total_time / sec_to_nsec - self.CLOCK_RES)
    ~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 0.49887036200016155 not greater than 0.499

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I will merge this, thanks everyone!