Properly handle labels without a "lang" in graph_extentities()

Previously, the "lang" value could be None because of getattr(...,
None). Subsequent usages of this value do not look quite right
(e.g. str(None) or set([None])). Instead, we now catch attribute error
and build the md5hash without the None value as well as insert an empty
set for language_code attribute when importing a label.

This also prepares for upgrade to python3.

cubicweb_skos/sobjects.py

@@ -181,12 +181,20 @@ def graph_extentities(graph):
         for rtype in label_rtypes:
             kind = rtype.split('_', 1)[0]  # drop '_label' suffix
             for label in extentity.values.pop(rtype):
-                lang = getattr(label, 'lang', None)
-                md5hash = md5(str(lang) + label.encode('utf-8'))
+                md5hash = md5()
+                language_code = set()
+                try:
+                    lang = label.lang
+                except AttributeError:
+                    pass
+                else:
+                    md5hash.update(lang.encode('utf-8'))
+                    language_code.add(lang)
+                md5hash.update(label.encode('utf-8'))
                 labelid = str(extentity.extid) + '#' + rtype + md5hash.hexdigest()
                 labels.append(ExtEntity('Label', labelid,
                                         {'label': set([label]),
-                                         'language_code': set([lang]),
+                                         'language_code': language_code,
                                          'kind': set([kind]),
                                          'label_of': set([extentity.extid])}))
         # yield extentity before labels since it must be handled first in case the ExternalUri