service: drop six import

This code is python3 only, so we don't need six.

BUG=None
TEST=CQ passes

Change-Id: If43597acbb361b99bb5714b1ec7a4766d6ee4e43
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2592667
Tested-by: Mike Frysinger <vapier@chromium.org>
Auto-Submit: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Alex Klein <saklein@chromium.org>
Commit-Queue: Alex Klein <saklein@chromium.org>
diff --git a/service/packages.py b/service/packages.py
index 5cb37c3..53ed741 100644
--- a/service/packages.py
+++ b/service/packages.py
@@ -15,8 +15,6 @@
 import re
 import sys
 
-import six
-
 from google.protobuf import json_format
 
 from chromite.api.gen.config import replication_config_pb2
@@ -676,7 +674,7 @@
   extra_env = None
   if useflags:
     new_flags = useflags
-    if not isinstance(useflags, six.string_types):
+    if not isinstance(useflags, str):
       new_flags = ' '.join(useflags)
 
     existing = os.environ.get('USE', '')