Use Literal consistently to annotate type hint, where string arguments come from a predefined input range.

[jvanhoefer]

Feature description

Since python 3.8 one can add a type hint of the form argument: Literal['a', 'b', 'c'], if a string argument comes from a predefined range of inputs. We could use this at many locations, where options are handed over as strings.

To be discussed before

• If we want to do this, we should do it consistently throughout pypesto, and not only at some random spots...
• We would drop python 3.6/7 support by doing that (python would not be importable in that python versions any more). We have to discuss python version support first.

[yannikschaelte]

not before #737