Update gnostic to yaml.v3: cleaner parsing, line number reporting
This release updates gnostic to use the yaml.v3 library and the yaml.Node class, which allows us to access line and column numbers of tokens during parsing.
This release upgrades the dependency only and does not include line number reporting.
This change will break clients that directly call auto-generated functions to create OpenAPIv2, OpenAPIv3, and Discovery structures. This is addressed in #195.