There is some incompatibility, like the gettext module in the g++ theme, the way themes are imported, unicode support, the print statement…
But maybe it's still possible to write a code that work on both versions?
I see. As a quick fix, I tried to call python 2.6 from the colout script, but that failed because importlib exists only in python3. I don't know how to make it work on both versions, I'm afraid.