Refactoring work

[2e0byo]

• Methods which don't use self should be static or classmethods.
  We have quite a lot of these in things like TextBlock.parse. Personally I'd also like to rename this .from_json.
• Remove sentinel None on class where not optional (over-use of optionals) (See also Spec of Session should be fully exposed on class #192)
• Set spec on class (see also Spec of Session should be fully exposed on class #192)
• Use ABCs to enforce consistent spec of objects, e.g. artist/album
• Config needs refactoring
• (easy: good first PR) we should clean up old syntax, e.g. inheriting from object, super(class, self) -> super()

[2e0byo]

Feel free to make suggestions/challenges here; let's link in refactoring issues so we can keep track of them.