Remove unused imports and fix some other linter errors.

recipe_engine/unittests/stream_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 cStringIO
-import os
-import sys
 import unittest
 
 import test_env
 
-from recipe_engine import recipe_api
 from recipe_engine import stream
 
 class StreamTest(unittest.TestCase):
   def _example(self, engine):
     foo = engine.make_step_stream('foo')
     foo.write_line('foo says hello to xyrself')
 
     bar = engine.make_step_stream('bar')
     foo.write_split('foo says hello to bar:\n  hi bar')
     bar.write_line('bar says hi, shyly')
@@ skipping 135 matching lines @@
 
   def test_buildbot_status_constraint(self):
     with stream.StreamEngineInvariants() as engine:
       foo = engine.make_step_stream('foo')
       foo.set_step_status('FAILURE')
       with self.assertRaises(AssertionError):
         foo.set_step_status('SUCCESS')
 
 if __name__ == '__main__':
   unittest.main()