v1.2.1: Improve public interface type signatures

• Use TypeVar self-type pattern (_ConfigSelf, _CVESelf) so Config and
  ConfigValidationError methods return the correct type for subclasses
• Use type(self)() instead of Config() in interpolate/copy/merge so
  subclass instances are preserved at runtime
• Widen input parameters from Dict to Mapping and List to Sequence
  where the parameter is read-only (overrides, updates, config params,
  section_order)
• Widen ConfigValidationError.config param to accept any Mapping
• Unify errors param type to Iterable[Mapping[str, Any]]