* Fix inheritance pb: there are some places where we filter constraints
  according to the class hierarchy, so rql unique constraint should'nt
  be neither a RQLContraint nor a RQLVocabularyConstraint subclass.
  Added test in unittest_schema reflecting this.

* So now we have to get explicitly RQLUniqueConstraint where desired (eg in
  server/hooks.py)

* Update i18ncubicweb command to include constraint types in generated pot file
  (this should have been there for a while...)

* Update unittest_schemaserial which has been broken when serializing format
  for rql constraints has changed