tracks2isrcs always wants a list of isrcs, though it mostly is 1 and pymb2 wanted 1

The signature of `tracks2isrcs` in `submit_isrcs` is `{ recording-id: [isrc1, isrc2, ...]}`.

We should additionally allow `{ recording-id: isrc}` since that is how it worked with python-musicbrainz2 and the natural way of gathering ISRCs is from disc, which should only yield at most 1 isrcs per track.

For PUIDs and echoprints single items are converted to single item lists internally already.