Switch to a different config format

The current ConfigParser spaghetti tries to mimick most of TOMLs behavior, with section categories (storage, pair) and type guessing.

ConfigObj doesn't seem to be an option because its "static" spec file doesn't work well with just passing all kwargs to storage classes, it probably doesn't get in the way, but doesn't solve any problems either.

What I'd basically need is something like the current INI-format, but with some kind of type system (i.e. like in JSON, a value unambiguously maps to a type, which probably means strings have to be quoted)

The options:

• TOML: Basically has everything I need, but extremely unstable IMO, and
  hardly a minimal config format. Python parsers exist, but AFAICT none of them
  are really well-written. The most popular one seems to be
  https://github.com/uiri/toml.

  Leftover issue: toml-lang/toml#271 especially troubling in our case when
  specifying paths and URLs.

• YAML: External dependency is huge, config format technically fulfills our needs, but is a very far
  departure from INI-like formats. It looks similar to Python, but in this context I don't like its syntax.

No file or test entry point is named. Start by reading the current ConfigParser-based configuration handling and comparing the stated TOML and YAML options, then define the required type behavior and compatibility scope before implementation; done means the selected format replaces the current INI-like parsing without losing the needed storage and pair options.