really remove all six imports

apparently, pyupgrade didn't know how to handle these...

beetsplug/replaygain.py

@@ -22,7 +22,7 @@ import subprocess
 import sys
 import warnings
 from multiprocessing.pool import ThreadPool, RUN
-from six.moves import queue
+import queue
 from threading import Thread, Event
 
 from beets import ui

test/helper.py

@@ -37,7 +37,7 @@ import shutil
 import subprocess
 from tempfile import mkdtemp, mkstemp
 from contextlib import contextmanager
-from six import StringIO
+from io import StringIO
 from enum import Enum
 
 import beets

test/test_importer.py

@@ -21,7 +21,7 @@ import shutil
 import unicodedata
 import sys
 import stat
-from six import StringIO
+from io import StringIO
 from tempfile import mkstemp
 from zipfile import ZipFile
 from tarfile import TarFile

test/test_logging.py

@@ -3,7 +3,7 @@
 import sys
 import threading
 import logging as log
-from six import StringIO
+from io import StringIO
 import unittest
 
 import beets.logging as blog

test/test_spotify.py

@@ -10,7 +10,7 @@ from beets import config
 from beets.library import Item
 from beetsplug import spotify
 from test.helper import TestHelper
-from six.moves.urllib.parse import parse_qs, urlparse
+from urllib.parse import parse_qs, urlparse
 
 
 class ArgumentsMock: