It appears that gramps-4.2.4 compiles fine with 3.5 as well. Just set: PYTHON_COMPAT=( python{3_4,3_5} )
Python 3.2 or greater is required.
commit e0ea92cdd970d1f4c0b532c65bf5b8a18232aa47 Author: Chris Mayo Date: Tue Oct 11 17:33:59 2016 +0000 app-misc/gramps: Support Python 3.5 and fix dependencies