[server] fix unicode conversion capability in UndoException By overriding the __unicode__ method, we ensure unicode builtin will return Unicode object even for python versions that don't have a __unicode__ method by default (python < 2.6) In previous versions, when this method is not defined, string conversion is attempted, and the result of string conversion is converted to Unicode using the system default encoding which is ascii the most of the time.

# type attribute might already be there if migrating from
# version < 3.5 to version >= 3.5.3, BaseTransition being added
# in bootstrap_migration
if versions_map['cubicweb'][0] >= (3, 5, 0):
    add_attribute('BaseTransition', 'type')
    sync_schema_props_perms('state_of')
    sync_schema_props_perms('transition_of')