add plumbing to produce a python testing image

[ajakk]

After running the new script manually and shelling into a container of the image:

eabf4e0e1ef8 / # ls -d /var/db/pkg/dev-*/{python,pypy}* | cat
/var/db/pkg/dev-lang/python-2.7.18_p16-r1
/var/db/pkg/dev-lang/python-3.10.12
/var/db/pkg/dev-lang/python-3.11.4
/var/db/pkg/dev-lang/python-3.12.0_beta3
/var/db/pkg/dev-lang/python-3.8.17
/var/db/pkg/dev-lang/python-3.9.17
/var/db/pkg/dev-lang/python-exec-2.4.10
/var/db/pkg/dev-lang/python-exec-conf-2.4.6
/var/db/pkg/dev-python/pypy-7.3.12
/var/db/pkg/dev-python/pypy-exe-bin-7.3.12
/var/db/pkg/dev-python/pypy3-7.3.12
/var/db/pkg/dev-python/pypy3_10-7.3.12-r1
/var/db/pkg/dev-python/pypy3_10-exe-bin-7.3.12
/var/db/pkg/dev-python/pypy3_9-7.3.12-r1
/var/db/pkg/dev-python/pypy3_9-exe-bin-7.3.12

[ajakk]

Agreed, it's probably worth simplifying in the future if we move past n=3, but didn't focus on that now since n=2 -> n=3 is relatively minimal overhead. Thanks all!

[mgorny]

meson-python may be fine without this: mesonbuild/meson-python#436 (comment).

I still think the official image would be better because otherwise we'd have to keep updating package list in meson-python.

That said, perhaps we should add some kind of metapackage to outsource this task to ::gentoo.