Remove unused imports and fix some other linter errors.

recipe_engine/stream_logdog.py

 # 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.
 
 """stream.StreamEngine implementation for LogDog, using Milo annotation
 protobuf.
 """
 
 import collections
-import contextlib
 import copy
 import datetime
 import functools
-import itertools
 import os
 import threading
 import sys
 
 from . import env
 from . import stream
-from . import util
 
-import google.protobuf.message
-import google.protobuf.timestamp_pb2 as timestamp_pb2
 import libs.logdog.bootstrap
 import libs.logdog.stream
 import libs.logdog.streamname
 import annotations_pb2 as pb
 
 
 # The datetime for the epoch.
 _EPOCH = datetime.datetime.utcfromtimestamp(0)
 
 # The annotation stream ContentType.
@@ skipping 776 matching lines @@
     return v.replace('/', '_')
 
   @staticmethod
   def _normalize(v):
     return libs.logdog.streamname.normalize(v, prefix='s_')
 
   def __str__(self):
     if not self._base:
       raise ValueError('Cannot generate string from empty StreamName.')
     return self._base