Status of cross-building Python extension modules

[Dima Kogan <dkogan@debian.org>]

Hi.

What's the status of this currently? I see some stuff on the wiki, but
it's not obviously up-to-date. I see that libpython3.9-dev is
Multi-Arch:same. And I was just able to cross-build an extension module
by installing libpython3.9-dev:foreign.

But since the package providing /usr/bin/python3 cannot be
Multi-Arch:same, I could not get the build flags with

  python3 -msysconfig

and I had to supply them manually. It looks like the flags I want live
in

  /usr/lib/python3.VERSION/_sysconfigdata__ARCH.py

which is available in Multi-Arch:same, so I COULD set up a build that
does everything automatically. Does there exist a "right" way to do this
today?

Also I see that python3-all-dev is not Multi-Arch:same, presumably
because it Depends:python3. So is this supposed to work today? If so,
what should the Build-Depends look like?

Thanks