Simplify typehint in tool use test case

Author: ncoghlan

tests/test_inference.py

@@ -245,7 +245,7 @@ def _append_fragment(f: LlmPredictionFragment, round_index: int) -> None:
         assert cloned_chat._messages == chat._messages
 
 
-def divide(numerator: float, denominator: float) -> float | str:
+def divide(numerator: float, denominator: float) -> float:
     """Divide the given numerator by the given denominator. Return the result."""
     return numerator / denominator