Remove unused imports and fix some other linter errors.

unittests/package_test.py

 #!/usr/bin/env python
 # Copyright 2015 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 import copy
 import doctest
 import os
 import subprocess
-import sys
 import unittest
 
 import repo_test_util
 
 import mock
 from recipe_engine import package
 
 
 TEST_AUTHOR = 'foo@example.com'
 
@@ skipping 450 matching lines @@
       self.assertFalse(checkout.called)
 
 
 def load_tests(_loader, tests, _ignore):
   tests.addTests(doctest.DocTestSuite(package))
   return tests
 
 
 if __name__ == '__main__':
   result = unittest.main()