Upgrade to numpy>=1.20 for typing support

[l3ku]

Numpy is pinned to version 1.19.2 in https://github.com/aws/sagemaker-xgboost-container/blob/master/requirements.txt#LL13C1-L13C14. This version doesn't yet include type hints in numpy.typing (https://numpy.org/devdocs/reference/typing.html), so it would be great to be able to use version >=1.20 in order to leverage type hints in the local development when using SageMaker script mode.