Fixes the new content revision test

tracim/tracim/tests/models/test_content_revision.py

 class TestContentRevision(BaseTest, TestStandard):
 
     def _new_from(self, revision):
-        excluded_columns = ('revision_id', '_sa_instance_state')
+        excluded_columns = ('revision_id', '_sa_instance_state', 'depot_file')
         revision_columns = [attr.key for attr in inspect(revision).attrs if not attr.key in excluded_columns]
         new_revision = ContentRevisionRO()