Update python3 sed hack to avoid '/usr/bin/python32' dep
In F28 and earlier /usr/bin/python exists during the build, and
the build process decides PYTHON is /usr/bin/python, so this
sed command works. In F29+ /usr/bin/python does not exist during
the build, so the build process decides /usr/bin/python2 is
PYTHON, and when we do 's!/usr/bin/python!/usr/bin/python3!', we
wind up with /usr/bin/python32...which is not what we wanted.
So, let's fix that.