- add missing ${MODPY_FLAVOR} to py-elftools BDEP
- this port doesn't use setuptoools to build; drop MODPY_SETUPTOOLS.
replace it with a plain BUILD_DEPENDS on setuptools, it's needed for
tools/binman/control.py which uses pkg_reseources from setuptools.
(I would guess that sometime in the future this might be dropped in
favour of using importlib.resources, added to core Python since 3.7,
but not yet).
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