Fix optional type for py3.9

tests/test_integration/test_inference.py

@@ -1,6 +1,7 @@
 import re
 import subprocess
 from pathlib import Path
+from typing import Optional
 
 import cv2
 import pooch
@@ -72,7 +73,7 @@ def test_detect_and_track_video(
     input_data_paths: dict,
     tmp_path: Path,
     flags_to_append: list,
-    no_timestamp_flag: str | None,
+    no_timestamp_flag: Optional[str],
 ):
     """Test the detect-and-track-video entry point when groundtruth is passed.