Drop support for python 2 and python < 3.8

[getzze]

Dropping support for old python versions will help to clean the code and to add type annotations.

Tasks:

• remove six from dependencies ( and replace six.text_type with str, etc. ) (Drop old python versions (<3.8) #1072)
• remove the -*- coding: utf-8 -*- lines (Drop old python versions (<3.8) #1072)
• add a pyproject.toml file. (Use pyproject.toml #1073)
• update outdated dependencies (Drop old python versions (<3.8) #1072)
• repair the tests (Repair tests (without recomputing most of the cassettes) #1074)

[ptrcnull]

this might include:

• removing six from dependencies ( and replacing six.text_type with str, etc. )
• removing the -*- coding: utf-8 -*- lines

[getzze]

also using a pyproject.toml file with all the relevant parameter for installing, testing and linting the files.

[getzze]

Done!