diff -Naur marshmallow-2.11.1.orig/marshmallow/schema.py test/marshmallow/schema.py
--- marshmallow-2.11.1.orig/marshmallow/schema.py	2017-01-08 22:23:48.000000000 +0100
+++ test/marshmallow/schema.py	2018-09-21 15:24:08.994234083 +0200
@@ -17,7 +17,7 @@
 from marshmallow.compat import (with_metaclass, iteritems, text_type,
                                 binary_type, OrderedDict)
 from marshmallow.exceptions import ValidationError
-from marshmallow.orderedset import OrderedSet
+from ordered_set import OrderedSet
 from marshmallow.decorators import (PRE_DUMP, POST_DUMP, PRE_LOAD, POST_LOAD,
                                     VALIDATES, VALIDATES_SCHEMA)
 from marshmallow.utils import missing