Latest version of itstool supports python 3 https://github.com/itstool/itstool/blob/2.0.4/NEWS Not sure what the standard procedure is for testing bumped packages, but I created a local 2.0.4 version of the ebuild, added python3_5 to PYTHON_COMPAT and emerged it without problems
itstool 2.0.4 is in the tree now. Closing (even though it lacks a python3_6 flag)