Make sure metadata lookup choices are a list to be able to add the optional marker (Python 3).

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

HISTORY.rst

@@ -22,6 +22,8 @@
 * Add separate Python 2 and Python 3 versions of the 
   MetadataType model .comma_splitter() static method.
 * Update the metadata app tests to work on Python 3.
+* Make sure metadata lookup choices are a list to be able to add the
+  optional marker (Python 3). 
 
 3.1.4 (2018-10-4)
 =================

mayan/apps/metadata/forms.py

@@ -62,7 +62,7 @@ class DocumentMetadataForm(forms.Form):
                         label=self.fields['value'].label
                     )
                     choices = self.metadata_type.get_lookup_values()
-                    choices = zip(choices, choices)
+                    choices = list(zip(choices, choices))
                     if not required:
                         choices.insert(0, ('', '------'))
                     self.fields['value'].choices = choices