Remove unused imports and fix some other linter errors.

recipe_engine/checker.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.
 
 """Implements a Checker object which can be used in place of `assert` to check
 conditions inside tests, but with much more debugging information, including
 a smart selection of local variables mentioned inside of the call to check."""
 
 import ast
 import copy
 import inspect
 import re
 import itertools
 
-from collections import OrderedDict, namedtuple, deque, defaultdict
+from collections import OrderedDict, deque, defaultdict
 
 from . import env
 import astunparse
 from expect_tests.type_definitions import CheckFrame, Check
 
 
 class _resolved(ast.AST):
   """_resolved is a fake AST node which represents a resolved sub-expression.
   It's used by _checkTransformer to replace portions of its AST with their
   resolved equivalents."""
@@ skipping 391 matching lines @@
 
 
 def _nameOfCallable(c):
   if hasattr(c, '__call__'):
     return c.__class__.__name__+'.__call__'
   if inspect.ismethod(c):
     return c.im_class.__name__+'.'+c.__name__
   if inspect.isfunction(c):
     return c.__name__
   return repr(c)