PyRFU's numpy (and cdflib) requirements are too high

[sapols]

PyRFU isn't compatible with the environment because its numpy requirements resolve to >=2.0.0,<2.1 which is too high for other packages. To be included, they'll need to offer backwards compatibility with numpy<2.

Their cdflib version could also be problematic; they require cdflib>=1.3.0, which isn't technically a problem on paper, but as mentioned in issue #3 it's possible some packages are advertising support beyond v1.0.0 when they don't mean to be—so we may eventually have to roll back our cdflib version if runtime errors are found. PyRFU would be playing nicer if they used the backwards compatibility trick in #3 to support cdflib<1 too.