Remove unused imports and fix some other linter errors.

recipe_engine/unittests/loader_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 os
-import sys
 import unittest
 
 import test_env
 
 from recipe_engine import loader, recipe_api, config
 import mock
 
 
 class TestRecipeScript(unittest.TestCase):
   def testReturnSchemaHasValidClass(self):
     with self.assertRaises(ValueError):
-      script = loader.RecipeScript({'RETURN_SCHEMA': 3}, 'test_script')
+      loader.RecipeScript({'RETURN_SCHEMA': 3}, 'test_script')
 
   def testRunChecksReturnType(self):
     sentinel = object()
     mocked_return = object()
     class FakeReturn(object):
       def as_jsonish(_self, hidden=sentinel):
         self.assertEqual(True, hidden)
 
         return mocked_return
 
@@ skipping 103 matching lines @@
 
     with mock.patch(
         'recipe_engine.loader._invoke_with_properties') as mocked_invoke:
       loader.invoke_with_properties(TestClass, None, None)
       args, _ = mocked_invoke.call_args
       self.assertTrue(['api', 'foo', 'bar'] in args)
 
 
 if __name__ == '__main__':
   unittest.main()