Chiming in here because this seems to have died without a resolution...
Going into abin, listing all the .py files, and running 2to3 -w on all of them seems to have resulted in a version of AFNI that works with Python 3.6 and 2.7 (well, it starts, certainly, which it didn't do before, although I haven't tested it extensively). As far as I can tell from perusing the output of 2to3, the only thing it really needed to change was print statements and excepts, which can be converted automatically with no problem.
Has anybody tried this and found it NOT to work? If there are problem files, I'm certainly happy to look at them to try to hand convert them.
Blaise