Remove unused imports and fix some other linter errors.

recipe_engine/unittests/loader_test.py

Index: recipe_engine/unittests/loader_test.py
diff --git a/recipe_engine/unittests/loader_test.py b/recipe_engine/unittests/loader_test.py
index 15f3585874ac8cdef6e45319e76f87c251aadfd0..fce45324a5810f118fe4736d2f50be88a5393dd2 100755
--- a/recipe_engine/unittests/loader_test.py
+++ b/recipe_engine/unittests/loader_test.py
@@ -3,8 +3,6 @@
 # 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
@@ -16,7 +14,7 @@ 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()