PyYAML has a SafeRepresenter in lib/... that defines

    def represent_unicode(self, data):
        return self.represent_scalar(u'tag:yaml.org,2002:str', data)

and a different SafeRepresenter in lib3/... that defines

    def represent_str(self, data):
        return self.represent_scalar('tag:yaml.org,2002:str', data)

so the right thing to do on Python 3 is to use represent_str.

(AnsibleUnicode is a subclass of six.text_type, i.e. 'str' on Python 3.)