Cross-building with python3-numpy

[Dima Kogan <dkogan@debian.org>]

Hi.

I'm trying to cross-build another one of my packages:

  https://salsa.debian.org/science-team/mrcal

I'm upstream here also. I had forgotten this, but apparently I need
numpy to build parts of this, not just to test it.

Unsurprisingly, added python3-numpy:any to the Build-Depends complains a
LOT.

Questions:

- Assuming the numpy package was fully set up for this, is right thing
  Build-Depends:python3-numpy:any ? The cross-building wiki page makes
  me think this is the case

- One part of being set-up for cross-building is that python3-numpy
  should be Multi-Arch:foreign. But how do I query this? "apt show
  python3-numpy" doesn't include the Multi-Arch tags.

- If somebody were to work on this, would the first step be to grab the
  python3-numpy sources, rebuild with the Multi-Arch:foreign tag, and
  then see if it works? I'm about to try that now. Is this the right way
  to do it?

Thanks!