Ditch support for python < 3.6

[novopl]

What?

Remove support for older python versions. Python2 is finally dead and moving to 3 means we can start with python3.6 support.

Why?

It's very wide spread and most new libraries target py36+ anyway, so there's already huge motivation to update to at least it (and it's backward compatible). The biggest drag was py2. We can then finally take use of native typing support and get rid of all those type comments from the codebase. It also means we can use the latest versions for all app dependencies.

How?

Has to be done over longer commit streak. There's a lot of fat that can be cut out and it's a good chance to clean up the code base in many ways (py3.6+ has many goodies on top of type annotations).