Python3

• $ sudo nano /etc/apt/sources.list
• # add
• deb http://ftp.de.debian.org/debian testing main

This line maybe not necessary 

• $ echo 'APT::Default-Release "stable";' | sudo tee -a /etc/apt/apt.conf.d/00local
• $ sudo apt-get update
• $ sudo apt-get -t testing install python3.6
• $ python3.6 -V