Force a SlackBuild to use python3

I am making a SlackBuild for a python package that is going to be a required dependency of another package I have. The issue I am running into is that it must be built using Python3. This is usually easy enough to accomplish by using "python3 setup.py install --root=$PKG". However not matter what I use the package is building against python2. Oddly enough if I skip the Slackbuild and manually build the package using "python3 setup.py install --root=/tmp/test" then it works fine. How can I force python3 on my SlackBuild script. The source in question is here...

https://github.com/vaab/colour

Thanks in advance.