python setup.py update_constants doesn't work

↪ python setup.py update_constants
running update_constants
/usr/bin/tx pull --force --resource=musicbrainz.attributes,musicbrainz.countries --source --language=none
Pulling translations for resource musicbrainz.attributes (source: po/attributes/attributes.pot)
-> en: po/attributes/attributes.pot
Pulling translations for resource musicbrainz.countries (source: po/countries/countries.pot)
-> en: po/countries/countries.pot
Done.
Parsing po/countries/countries.pot
Traceback (most recent call last):
File "setup.py", line 781, in <module>
setup(**args)
File "/usr/lib/python3.6/distutils/core.py", line 148, in setup
dist.run_commands()
File "/usr/lib/python3.6/distutils/dist.py", line 955, in run_commands
self.run_command(cmd)
File "/usr/lib/python3.6/distutils/dist.py", line 974, in run_command
cmd_obj.run()
File "setup.py", line 496, in run
if not message.id or not isinstance(message.id, unicode):
NameError: name 'unicode' is not defined