As discussed over the mailing list, reformat code using Black. This is a
one-off change to reformat all our codebase. Moving forward, we
shouldn't expect such blanket reformats. Rather, we expect each change
to already comply with the Black formatting style.

`mapping.get_sequence(...).as_str_list()` is a very common
pattern seen both in plugins and the core. Adding a helper to reduce
the number of operations will make usage smoother

Now that both are equivalent, we can skip the sanitization part before
the yaml call.

- Adding 'get_sequence' on MappingNode to access sequences in a mapping
- Adding 'sequence_at' on SequenceNode to access sequences in a sequence
- Adding 'mapping_at' on SequenceNode to access mappings in a sequence

Using "*_at" in sequences allows us to quickly understand if we are
dealing with a sequence or a mapping.

- 'get_scalar()' allows retrieving a scalar node from a mapping.
- 'as_str()' casts a ScalarNode into a string (thus removing the node
  information).

Both together, those replace 'node_get(mapping, key, type=str)' but
also allow retrieving the 'Node' itself, which will allow in the future
lazier provenance computation.

Replace YAML internals with a new Node type, and refactor everything
to use it cleanly.

This work was also by James Ennis <james.ennis@codethink.co.uk>

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

Fixes #331.