[py3] Fixed __unicode__ methods missed in d4a0b278

# Using to_field on the through model

@python_2_unicode_compatible

class Car(models.Model):

    make = models.CharField(max_length=20, unique=True)

    drivers = models.ManyToManyField('Driver', through='CarDriver')

    def __unicode__(self, ):

    def __str__(self):

        return self.make

@python_2_unicode_compatible

class Driver(models.Model):

    name = models.CharField(max_length=20, unique=True)

    def __unicode__(self, ):

    def __str__(self):

        return self.name

@python_2_unicode_compatible

class CarDriver(models.Model):

    car = models.ForeignKey('Car', to_field='make')

    driver = models.ForeignKey('Driver', to_field='name')

    def __unicode__(self, ):

    def __str__(self):

        return "pk=%s car=%s driver=%s" % (str(self.pk), self.car, self.driver)