Fixed #23850 -- Fixed a migrations test failure on Mac OS X & Python 3

from __future__ import unicode_literals

import codecs

import importlib

import os

import shutil

        content = cmd("0001", migration_name_0001)

        self.assertIn("dependencies=[\n]", content)

        # Python 3.3+ importlib caches os.listdir() on some platforms like

        # Mac OS X (#23850).

        if hasattr(importlib, 'invalidate_caches'):

            importlib.invalidate_caches()

        # generate an empty migration

        migration_name_0002 = "my_custom_migration"

        content = cmd("0002", migration_name_0002, "--empty")