devserver: module import cleanup

* Import statements re-grouped: standard lib, third party,
  project-local; separated by a single line, preceded by a single line
  and followed by two lines. Each group is sorted alphabetically.

* Removed unused imports.

BUG=None
TEST=Passes unit tests.

Change-Id: Iee4f78b1256457bd19b2c159a29763ba3404c272
Reviewed-on: https://gerrit.chromium.org/gerrit/33859
Reviewed-by: Chris Sosa <sosa@chromium.org>
Commit-Ready: Gilad Arnold <garnold@chromium.org>
Tested-by: Gilad Arnold <garnold@chromium.org>
diff --git a/build_artifact_unittest.py b/build_artifact_unittest.py
index 4ef347d..119f6d3 100755
--- a/build_artifact_unittest.py
+++ b/build_artifact_unittest.py
@@ -11,15 +11,17 @@
 run these unittests from within the chroot.  The tools are self-explanatory.
 """
 
-import mox
 import os
 import shutil
 import subprocess
 import tempfile
 import unittest
 
+import mox
+
 import build_artifact
 
+
 _TEST_GOLO_ARCHIVE = (
     'gs://chromeos-image-archive/x86-alex-release/R19-2003.0.0-a1-b1819')
 _TEST_SUITES_TAR = '/'.join([_TEST_GOLO_ARCHIVE,