samdivaio

Felix Krull runs a PPA offering basically any version of Python (seriously, there is 2.3.7 build for vivid...) for many Ubuntu releases at

https://launchpad.net/~fkrull/+archive/ubuntu/deadsnakes

Do the usual:

sudo add-apt-repository ppa:fkrull/deadsnakes
sudo apt-get update
sudo apt-get install python3.5
It will not overwrite your existing python3.4 which is still symlinked as python3. If you want python3.5 to be the default python3, change the symlink