Maintain python 2 compatibility

Jellyfish is no longer python 2 compatible as of release 0.7.0.

By pinning the previous release, beets is still able to be installed
and run on python 2 systems without issue.

docs/changelog.rst

@@ -67,6 +67,7 @@ Changes:
 
 Fixes:
 
+* Pin jellyfish requirement to version 0.6.0 to maintain python 2 compatibility.
 * A new importer option, :ref:`ignore_data_tracks`, lets you skip audio tracks
   contained in data files :bug:`3021`
 * Restore iTunes Store album art source, and remove the dependency on

setup.py

@@ -92,7 +92,7 @@ setup(
         'unidecode',
         'musicbrainzngs>=0.4',
         'pyyaml',
-        'jellyfish',
+        'jellyfish==0.6.0',
     ] + (['colorama'] if (sys.platform == 'win32') else []) +
         (['enum34>=1.0.4'] if sys.version_info < (3, 4, 0) else []),