Fix Python 3.9 compatibility in conftest

Use explicit Union[..., None] instead of | None or Optional[].
The | operator for union types is only supported in Python 3.10+.
Union syntax works across all Python versions.

Author: ctoth

tests/conftest.py

@@ -54,6 +54,6 @@ def _assert_positions(text: str, nodes: list[dict[str, Any]]) -> None:
     return _assert_positions
 
 
-def convert(html: str, callback: Callable[..., dict[str, Union[str, int]]] | None = None, file: str = "") -> str:
+def convert(html: str, callback: Union[Callable[..., dict[str, Union[str, int]]], None] = None, file: str = "") -> str:
     """Helper to convert HTML to text with optional callback and file parameter."""
     return html_to_text(html, node_parsed_callback=callback, file=file)