Remove unused imports and fix some other linter errors.

recipe_engine/step_runner.py

 # 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 StringIO
+import calendar

[Vadim Sh., 2017/02/21 05:46:26]

used on line 388, but wasn't imported

 import collections
 import contextlib
 import datetime
 import json
 import os
 import re
+import StringIO
 import sys
+import tempfile
 import time
-import tempfile
 import traceback
 
 from . import recipe_api
 from . import recipe_test_api
 from . import stream
 from . import types
 from . import util
 
 import subprocess42
 
@@ skipping 651 matching lines @@
   supplied command, and only uses the |env| kwarg for modifying the environment
   of the child process.
   """
   saved_path = os.environ['PATH']
   try:
     if path is not None:
       os.environ['PATH'] = path
     yield
   finally:
     os.environ['PATH'] = saved_path