Handle older numpy versions / py 3.8 in VecND extensions

[teh-cmc]

What the title says.

This is what prevented us from doing the logical thing in Arrow3D extensions (and became even more of an issue after I made the PointND components use VecND internally in #2894).

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[abey79]

Looks good 👍🏻 Annoying to have to do this—I won't be missing 3.8 when we drop it...