avoid NumPy 1.21.0 due to bug

[grlee77]

This PR blocks NumPy 1.21.0 due to numpy/numpy#19325

I cherry-picked this commit from #789 where it resolved the observed segfault

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/tutorial.rst
• Changes documented in docs/release.rst
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[joshmoore]

I think the stuck jobs are due to the fact that the workflow definition changed. I'm merging so that other PRs can be updated to go green. (cc: @grlee77 @clbarnes)