Install python 3.6 for building HTTPS-Everywhere

The next HTTPSE release, to be released on monday, is converting all the toolings to python 3.6, including the components of the build script. To be able to build HTTPSE, we will need to install python 3.6.

To do that, we can either use a debian buster (testing) container, or build python 3.6 from sources.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information