Debian Jessie does not come with the correct Python version out of the box, and instead comes with Python 2. To be able to install Python 3(.5), we have a few options. We could build and install from source, but as per Debian's website, we shouldn't do this. Instead, we will use pyenv, a tool that allows users to switch Python versions easily without breaking their system.
To install pyenv, we will use the official installer.
curl -L https://raw.githubusercontent.com/yyuu/pyenv-installer/master/bin/pyenv-installer | bash