Replace deprecated usage of np.asscalar with np.ndarray.item()

[`numpy.asscalar()` is deprecated since version 1.16](https://github.com/numpy/numpy/blob/master/numpy/lib/type_check.py#L519-L548).

This PR replaces its usage with [`numpy.ndarray.item()`](https://www.numpy.org/devdocs/reference/generated/numpy.ndarray.item.html)