[predicates] keep making `yes` predicate importable from cubicweb (closes #2790319) The `yes` predicate was moved out in CubicWeb 3.15 but it should have remained importable from the cubicweb.predicates module, for bw compat reasons and also because logilab.common.registry is an implementation detail.

import ConfigParser
try:
    undo_actions = config.cfgfile_parser.get('MAIN', 'undo-support', False)
except ConfigParser.NoOptionError:
    pass # this conf. file was probably already migrated
else:
    config.global_set_option('undo-enabled', bool(undo_actions))