Remove unused imports and fix some other linter errors.

recipe_engine/loader.py

Index: recipe_engine/loader.py
diff --git a/recipe_engine/loader.py b/recipe_engine/loader.py
index 12845aabd86a9480ed6b14e3b146b59b96e7e6cc..8f26265d4b94af2897448db6f2149525a73803f6 100644
--- a/recipe_engine/loader.py
+++ b/recipe_engine/loader.py
@@ -3,7 +3,6 @@
 # that can be found in the LICENSE file.
 
 import collections
-import contextlib
 import imp
 import inspect
 import os
@@ -15,9 +14,9 @@ from .config import ConfigContext, ConfigGroupSchema
 from .config_types import Path, ModuleBasePath, PackageRepoBasePath
 from .config_types import RecipeScriptBasePath
 from .config_types import RECIPE_MODULE_PREFIX
-from .recipe_api import RecipeApi, RecipeApiPlain, RecipeScriptApi
+from .recipe_api import RecipeApiPlain, RecipeScriptApi
 from .recipe_api import _UnresolvedRequirement
-from .recipe_api import Property, BoundProperty
+from .recipe_api import BoundProperty
 from .recipe_api import UndefinedPropertyException, PROPERTY_SENTINEL
 from .recipe_test_api import RecipeTestApi, DisabledTestData