fix leaking imports in minimal testing

tests/datasets/test_streaming_video_encoder.py

@@ -20,10 +20,13 @@ import queue
 import threading
 from unittest.mock import patch
 
-import av
 import numpy as np
 import pytest
 
+pytest.importorskip("av", reason="av is required (install lerobot[dataset])")
+
+import av  # noqa: E402
+
 from lerobot.datasets.video_utils import (
     VALID_VIDEO_CODECS,
     StreamingVideoEncoder,