Fix import error

See also:
https://github.com/edx/edx-platform/pull/9969

I'm not at a linux box to do a rebase from the command line, so I'm creating a new PR.

common/lib/xmodule/xmodule/modulestore/tests/factories.py

@@ -7,7 +7,7 @@ import pymongo.message
 import threading
 import traceback
 from collections import defaultdict
-from decorator import contextmanager
+from contextlib import contextmanager
 from uuid import uuid4
 
 from factory import Factory, Sequence, lazy_attribute_sequence, lazy_attribute