Fix a test that relied on an exception outliving the `except` block, which doesn't happen on py3k.

    class MultipleAutoFields(models.Model):

        auto1 = models.AutoField(primary_key=True)

        auto2 = models.AutoField(primary_key=True)

except AssertionError as assertion_error:

    pass # Fail silently

except AssertionError as exc:

    assertion_error = exc

assert str(assertion_error) == "A model can't have more than one AutoField."