Lower NumPy version requirement

[gsganden]

We are using a numpy.typing module that requires numpy>1.20, which is higher than we need elsewhere. I don't think that module is not adding enough value to warrant limiting the portability of the library.

It should be OK to ignore the notebooks in this review.

PR Checklist

Recommended steps to avoid delays in CI and code review:

• [x] Use nbqa black nbs to use black to format the notebook code.
• [x] Use nbqa flake8 nbs to run linting checks on the notebooks.

Required steps:

• [x] Increment version number in settings.ini if changing the library.
• [x] Use nbdev_preview to preview the documentation site if changing the notebooks.
• [x] Use nbdev_prepare to export, test, and clean notebooks.

[review-notebook-app[bot]]

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