Implement zenodio.metadata to mediate Zenodo's API with local YAML metadata

Zenodio is a Python package we’re building to interact with Zenodo. For our various doc/technote/publishing projects we want to use YAML files (embedded in a Git repository, for example) to maintain deposition metadata so that the upload process itself can be automated.

The zenodio.metadata sub package provides a Python representation of Zenodo metadata (but not File or Zenodo deposition metadata).

See DM-4725 for the upload API work, which consumes the metadata objects.