...just moments after I posted my modification (which is entirely focussed on setup.py):
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=292197

I don't know intricacies of python packaging or even how it's usually done in FreeBSD, but it should be pretty simple.  The non-python parts (built via meson) didn't change significantly in terms of build system since 5.x.  And after that you need basic python package install - in some distros it's even automatic.  setup.py should work, or alternatively pyproject.

The only really weird thing I know about is that in a (conceptually) single package you have both "python package" and a meson-based non-python package (though e.g. for nixos.org it gets packaged as two builds, one depending on another).

--Vladimir