fix python 3 bytes issue

bob/learn/tensorflow/test/data/dummy_verify_config.py

@@ -21,5 +21,5 @@ def reader(biofile):
     data = read_original_data(
         biofile, database.original_directory, database.original_extension)
     label = file_to_label(biofile)
-    key = str(biofile.path)
+    key = str(biofile.path).encode("utf-8")
     return (data, label, key)