Remove unused imports and fix some other linter errors.

recipe_engine/unittests/config_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
+from recipe_engine import config
 
 class TestConfigGroupSchema(unittest.TestCase):
   def testNewReturnsConfigGroup(self):
     schema = config.ConfigGroupSchema(test=config.Single(int))
 
     self.assertIsInstance(schema.new(test=3), config.ConfigGroup)
 
   def testCallCallsNew(self):
     schema = config.ConfigGroupSchema(test=config.Single(int))
     sentinel = object()
@@ skipping 14 matching lines @@
     schema = config.ConfigGroupSchema(test=config.Enum('foo', 'bar'))
     self.assertIsInstance(schema.new(test='foo'), config.ConfigGroup)
 
   def testMustBeOneOf(self):
     schema = config.ConfigGroupSchema(test=config.Enum('foo', 'bar'))
     with self.assertRaises(ValueError):
       schema.new(test='baz')
 
 if __name__ == '__main__':
   unittest.main()