Default instalation seems to be inconsistent a little when PYTHON variable is not used during build. Python 2 versions of selinux and semanage modules are built and installed, but /usr/sbin/semanage and other tools are already changed to use /usr/bin/python3. I'd like to propose to change to Python 3 by default and leave it on a maintainer to use PYTHON=/usr/bin/python2 when they doesn't have Python 3 available. They would also need to change shebangs of some tools, but this is the same situation as used to be when default was Python 2 and maintainers needed to change shebangs to /usr/bin/python3. Do you have any objections?
