Pin numpy<2.0 for PyPI install issue

[anissa111]

PR Summary

Temporary fix for #598

So, due to a pip bug (see Unidata/cftime#327), sometimes pre-release versions get installed, which is what's messing with our release tests. Since numpy 2.0 isn't actually released yet, I don't anticipate any problems with this pin until numpy 2.0 is fully released and generally compatible.

PR Checklist

General

• Make an issue if one doesn't already exist
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