Remove unused imports and fix some other linter errors.

recipe_engine/unittests/fetch_test.py

 #!/usr/bin/env python
 # Copyright 2016 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 base64
-import io
 import json
-import os
-import sys
 import unittest
-import time
 
 import test_env
 
 import mock
 import subprocess42
 
 from recipe_engine import fetch
 from recipe_engine import requests_ssl
 
 
@@ skipping 321 matching lines @@
       'author': 'author',
       'message': 'message',
     })
     self.assertEqual(counts['sleeps'], 4)
     requests_get.assert_has_calls([
       mock.call('repo/+/revision?format=JSON'),
     ] * 5)
 
 if __name__ == '__main__':
   unittest.main()