musicbrainz.py: Fix undefined name in submit_barcodes

Signed-off-by: Simon Chopin <chopin.simon@gmail.com>
Merge remote-tracking branch 'laarmen/fix-undefined-name'

Signed-off-by: Johannes Dewender <github@JonnyJD.net>
ignore docs/_build for setup.py build

Otherwise local building fails when the documentation was previously built.

Signed-off-by: Johannes Dewender <github@JonnyJD.net>