MaxLength returns wrong length for unicode strings on Python 2.7

[onyxfish]

Test:

    def test_max_length_unicode(self):
        rows = [
            ['a'],
            [u'👍'],
            ['w']
        ]

        table = Table(rows, ['test'], [Text()])
        MaxLength('test').validate(table)
        self.assertEqual(MaxLength('test').run(table), 1)
        self.assertIsInstance(MaxLength('test').run(table), Decimal)