tidy: Remove no longer needed np-special casing

[danra]

np.astype became available in numpy 2.1

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[danra]

Looks like this is failing due to dask not having implemented np.astype. Not familiar with dask. Shall I just close this, or should this be amended somehow to only use np.astype in the numpy, non-dask case?

[keewis]

it's not just that: we're still maintaining backwards compatibility with numpy<2 (see the bare-minimum CI), which means that we still need the special-casing for that, as well.

Either way, thanks for the PR. Unfortunately, I think this might not be the right time, yet.

Edit: but maybe you can repurpose the PR to make the comment more accurate? I.e. that this is for backwards-compat and dask?