To get rid of Python2.7, I inspect some ebuilds DEPEND on this. In case of Teeworlds, the teeworlds-0.6.4.ebuild contain the line PYTHON_COMPAT=( python2_7 ) But in the upstream documentation (https://www.teeworlds.com/?page=docs&wiki=compiling_everything_linux) on may read: "Note: Teeworlds 0.5.2 and earlier requires python 2.x to compile. Python 3.x will not work. Python 3.x support is introduced with Teeworlds 0.6.0." As a test, I changed the set to PYTHON_COMPAT=( python3_5 python3_6 ) and the ebuild run for me without any problem.
It's had Python 3 for a while, but in today's commit, we gained 6, 7, 8, 9: https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=756334db7d87f8a52c2d48302b88c39b613d51f4.