As of September 2017 Raspbian does not yet include Python 3.6. To install it you need to compile the source code.
a1. Install the required build-tools (some might already be installed on your system).
sudo apt-get update sudo apt-get install build-essential tk-dev libncurses5-dev libncursesw5-dev libreadline6-dev libdb5.3-dev libgdbm-dev libsqlite3-dev libssl-dev libbz2-dev libexpat1-dev liblzma-dev zlib1g-dev
wget https://www.python.org/ftp/python/3.6.2/Python-3.6.2.tar.xz tar xf Python-3.6.2.tar.xz cd Python-3.6.2 ./configure --enable-optimizations make sudo make altinstall
You can now delete the source code and uninstall the previously installed packages. When uninstalling the packages, make sure you only remove those that were not previously installed on your system. Also, remember to adjust version numbers if necesarry.
.. code-block:: bash
sudo rm -r Python-3.6.0 rm Python-3.6.0.tgz sudo apt-get --purge remove build-essential tk-dev sudo apt-get --purge remove libncurses5-dev libncursesw5-dev libreadline6-dev sudo apt-get --purge remove libdb5.3-dev libgdbm-dev libsqlite3-dev libssl-dev sudo apt-get --purge remove libbz2-dev libexpat1-dev liblzma-dev zlib1g-dev sudo apt-get autoremove sudo apt-get clean