python-rpm-macros is not actually used here, `%python3_build` is not it.
Also `Group` has *not* been abolished.

`Group` has *been* been abolished.

python3_build does:
#rpm -q --eval=%python3_build

last_flavor=`[ -f _current_flavor ] && cat _current_flavor || true`
if [ -z "$last_flavor" ]; then last_flavor="tmp"; fi
if [ "$last_flavor" != "python3" ]; then
if [ -d build ]; then mv build _build.$last_flavor; fi
if [ -d _build.python3 ]; then mv _build.python3 build; fi
fi
echo python3 > _current_flavor
python_flavor=python3

/usr/bin/python3 setup.py build \
--executable="/usr/bin/python3 -s"

Where python3_build is defined in /usr/lib/rpm/macros.d/macros.python_all

Which comes from python-rpm-macros

Which while pulled by other python dependencies now is not guaranteed to get pulled.