Import CMS code using fully qualified module names

or, using proper the proper Python 3 relative imports
(e.g., 'from .views import x') which are standard and
unambiguous.

cms/djangoapps/contentstore/tests/utils.py

@@ -13,7 +13,7 @@ from django.test.client import Client
 from mock import Mock
 from opaque_keys.edx.keys import AssetKey, CourseKey
 
-from contentstore.utils import reverse_url
+from cms.djangoapps.contentstore.utils import reverse_url
 from student.models import Registration
 from xmodule.contentstore.django import contentstore
 from xmodule.modulestore import ModuleStoreEnum