clean up usage of the six module

Now that we only support Python 3, we can drop all six usage.

BUG=b:187789302
TEST=CQ passes

Change-Id: I92ec7022f1f92fccc7d82ef3f262557081c0fc1b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2910860
Tested-by: Mike Frysinger <vapier@chromium.org>
Auto-Submit: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Chris McDonald <cjmcdonald@chromium.org>
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
diff --git a/scripts/pushimage.py b/scripts/pushimage.py
index 4e5b449..8fd43be 100644
--- a/scripts/pushimage.py
+++ b/scripts/pushimage.py
@@ -8,14 +8,13 @@
 artifacts for signing (which a signing process will look for).
 """
 
+import configparser
 import getpass
+import io
 import os
 import re
 import textwrap
 
-from six.moves import configparser
-from six.moves import StringIO
-
 from chromite.lib import constants
 from chromite.lib import commandline
 from chromite.lib import cros_build_lib
@@ -195,7 +194,7 @@
     https://bugs.python.org/issue16058
     """
     # Write the current config to a string io object.
-    data = StringIO()
+    data = io.StringIO()
     config.write(data)
     data.seek(0)
 
@@ -246,7 +245,7 @@
     for alt in self.GetAltInsnSets():
       config.remove_section(alt)
 
-    output = StringIO()
+    output = io.StringIO()
     config.write(output)
     data = output.getvalue()
     osutils.WriteFile(output_file, data)