Promote Series information to main Release information

[VinylVault]

I know that series information is available via fetch, but it would be useful if this was available (or null) at the top level of a release

Original discussion: #150

[JOJ0]

This should be easy enough to implement. Similar to this one: https://github.com/joalla/discogs_client/pull/119/files but I'm not sure if Series could be ListField or rather be an ObjectField? What does the API for series return? Is it always a list of JSON objects? Or could it also be one JSON object only? Some raw API output would help to find out @VinylVault

ListField example: https://github.com/joalla/discogs_client/blob/master/discogs_client/models.py#L525

ObjectField example https://github.com/joalla/discogs_client/blob/master/discogs_client/models.py#L527

You want to draft a PR @VinylVault ? I'm happy to review and help then.

[VinylVault]

There is only ever on Series, that I've come across, but that isn't necessarily the case for every release ... probably best to treat is like an artist or record label (seeing as they are stored as labels and URL'd as labels [don't as me why])