Numpy dependency version needs to be bumped to 1.20.x for numpy.typing

Or else the typing import needs to be removed (it isn't strictly necessary for any functionality).

I can open a PR for this, it's pretty simple, but I think someone needs to answer the question of whether a dependency version bump or the need for typing is more important.

https://github.com/facebook/prophet/blob/c00f6a2d72229faa6acee8292bc01e14f16f599c/python/prophet/forecaster.py#L18

on np<1.20, an error occurs, module not found.