Limit numpy version to 1.20 to prevent numba error about incompatibility with numpy

[lneuhaus]

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[josh146]

Hey @lneuhaus! Thanks for this. Instead of limiting NumPy, can we instead limit numba<0.54?

I think better to restrict Numba (a package less users might already have installed) than NumPy, to avoid potential dependency resolving issues

[josh146]

Note: I've already made this change in #278