You could add a check for the python version in the venv before wiping it. If it’s 2, wipe it, if it’s 3, don’t wipe.
Something like this should do it
if [[ -f %{prefix}/%{name}/.venv/python2 ]]; then
echo "Python2 found. Wiping venv"
rm -rf %{prefix}/%{name}/.venv
fi