code: def test_mp4_writing(tmp_path, test_images):
      frames = iio.imread(test_images / "newtonscradle.gif")
  
      mp4_bytes = iio.imwrite(
          "<bytes>",
          frames,
          extension=".mp4",
          plugin="pyav",
          codec="libx264",
      )
  
      # libx264 writing is not deterministic and RGB -> YUV is lossy
      # so I have no good ideas how to do serious assertions on the file
      assert mp4_bytes is not None
  
imports: import io
  import warnings
  from contextlib import ExitStack
  from pathlib import Path
  import numpy
  import pytest
  from conftest import IS_PYPY
  import imageio.v3
  from av.video.format import names
  from imageio.plugins.pyav import _format_to_dtype
label_desc: Use image node_id 3 for calling a global function with example usage: test_mp4_writing(tmp_path, test_images) without return types