Refactor per requirement options

[sbidoul]

Some refactoring and readability improvements towards #11325

[sbidoul]

Rebased to have a green build.

[q0w]

Maybe it should be Sequence[str] instead of Union[str, List[str]]?

[pradyunsg]

Maybe it should be Sequence[str] instead of Union[str, List[str]]?

That'd rely on the fact that str is a Sequence, and it doesn't clearly indicate that "string" is a valid input -- whereas "string" and ["string", "string2"] are valid inputs here and that's clearly indicated by the union here.