On my amd64 arch machine I;
emerged python 3.5
eselected python 3.5
edited make.conf to set PYTHON_TARGETS to "python2_7 python3_5"
running emerge -pv --depclean =python-3.4.5 to see what needs to be rebuilt
Then tryed to rebuild those packages to allow removal of 3.4, however, it looks like that I would then have to change PYTHON_SINGLE_TARGET to 3.5 too, and some other packages still require it to be set to 2.7, so i've bailed out of trying to get rid of 3.4 on that box. I'll leave PYTHON_TARGETS at "python2_7 python3_5" unless I find something that also needs 3.4 in there.

Failure came fast, example;

The following REQUIRED_USE flag constraints are unsatisfied:
python? ( at-most-one-of ( python_targets_python3_4 python_targets_python3_5 python_targets_python3_6 )

So ive unset PYTHON_TARGETS and PYTHON_SINGLE_TARGET again.