Silence spurious mypy error

Numpy typing definitely still needs work, but I think this error has appeared as a result of changes in dependencies, and isn't real.

`np.asarray` definitely returns an ndarray - though it is possible `np.ndarray` is
not the right type annotation to use.

Author: rwb27

src/labthings_fastapi/types/numpy.py

@@ -65,7 +65,7 @@ def np_to_listoflists(arr: np.ndarray) -> NestedListOfNumbers:
 
 def listoflists_to_np(lol: Union[NestedListOfNumbers, np.ndarray]) -> np.ndarray:
     """Convert a list of lists to a numpy array (or pass-through ndarrays)"""
-    return np.asarray(lol)
+    return np.asarray(lol)  # type: ignore[return-type]
 
 
 # Define an annotated type so Pydantic can cope with numpy