升级到Django 1.9后的时区问题

我在Django 1.8.5中通过了一项测试。这是

def test_user_returns_from_vacation_correctly_increments_review_timestamps(self):
        self.user.profile.on_vacation = True

        now = timezone.now()
        an_hour_ago = now - timedelta(hours=1)
        two_hours_ago = now - timedelta(hours=2)

        self.user.profile.vacation_date = an_hour_ago
        self.user.profile.save()
        self.review.last_studied = two_hours_ago
        self.review.save()
        previously_studied = self.review.last_studied

        user_returns_from_vacation(self.user)
        rev = UserSpecific.objects.get(id=self.review.id) #<---This is the line causing the error!
        print(rev)
        self.assertNotEqual(rev.last_studied, previously_studied)
        self.assertAlmostEqual(rev.last_studied, an_hour_ago, delta=timedelta(seconds=1))

但是，在升级到Django 1.9时，此测试会导致以下错误：

Error
Traceback (most recent call last):
/pytz/tzinfo.py", line 304, in localize
etc etc
    raise ValueError('Not naive datetime (tzinfo is already set)')
ValueError: Not naive datetime (tzinfo is already set)

以下是相关user_returns_from_vacation代码：

def user_returns_from_vacation(user):
    """
    Called when a user disables vacation mode. A one-time pass through their reviews in order to correct their last_studied_date, and quickly run an SRS run to determine which reviews currently need to be looked at.
    """
    logger.info("{} has returned from vacation!".format(user.username))
    vacation_date = user.profile.vacation_date
    if vacation_date:
        users_reviews = UserSpecific.objects.filter(user=user)
        elapsed_vacation_time = timezone.now() - vacation_date
        logger.info("User {} has been gone for timedelta: {}".format(user.username, str(elapsed_vacation_time)))
        updated_count = users_reviews.update(last_studied=F('last_studied') + elapsed_vacation_time)
        users_reviews.update(next_review_date=F('next_review_date') + elapsed_vacation_time)
        logger.info("brought {} reviews out of hibernation for {}".format(updated_count, user.username))

    user.profile.vacation_date = None
    user.profile.on_vacation = False
    user.profile.save()

更新：如果我删除F（）表达式，并用for循环替换它们，此错误将消失。像这样：

    for rev in users_reviews:
        lst = rev.last_studied
        nsd = rev.next_review_date
        rev.last_studied = lst + elapsed_vacation_time
        rev.next_review_date = nsd + elapsed_vacation_time
        rev.save()

我在1.9的发行说明中注意到，他们已经为时区纯朴的数据库添加了TIME_ZONE属性（例如SQLite，这是我运行测试所使用的属性）。我尝试将我的时区添加到数据库配置中，但是问题仍然存在。有任何想法吗？这是我在settings.py中与时区相关的设置

MY_TIME_ZONE = 'America/New_York'
TIME_ZONE = MY_TIME_ZONE
DATABASES = {
        'default': {
            'ENGINE': 'django.db.backends.sqlite3',
            'NAME': os.path.join(BASE_DIR, 'db.sqlite3'),
            'TIME_ZONE': MY_TIME_ZONE
        }