old_contents: import datetime
  
  from django.apps import AppConfig
  from django.core.checks import Error, register
  
  
  class BoardingHouseDemoConfig(AppConfig):
      name = 'boardinghouse.contrib.demo'
  
      def ready(self):
          # Make sure our required setting exists.
          from django.apps import apps
          from django.conf import settings
  
          if not hasattr(settings, 'BOARDINGHOUSE_DEMO_PREFIX'):
              settings.BOARDINGHOUSE_DEMO_PREFIX = '__demo_'
  
          if not hasattr(settings, 'BOARDINGHOUSE_DEMO_PERIOD'):
              settings.BOARDINGHOUSE_DEMO_PERIOD = datetime.timedelta(31)
  
          from boardinghouse.contrib.demo import receivers  # NOQA
  
  
  @register('settings')
  def check_demo_prefix_stats_with_underscore(app_configs=None, **kwargs):
      from django.conf import settings
  
      if not settings.BOARDINGHOUSE_DEMO_PREFIX.startswith('_'):
          return [Error('BOARDINGHOUSE_DEMO_PREFIX must start with an underscore',
                        id='boardinghouse.contrib.demo.E001')]
  
      return []
  
  
  @register('settings')
  def check_demo_expiry_is_timedelta(app_configs=None, **kwargs):
      from django.conf import settings
  
      if not isinstance(settings.BOARDINGHOUSE_DEMO_PERIOD, datetime.timedelta):
          return [Error('BOARDINGHOUSE_DEMO_PERIOD must be a datetime.timedelta() instance',
                        id='boardinghouse.contrib.demo.E002')]
  
      return []
  
  
  @register('settings')
  def ensure_contrib_template_installed(app_configs=None, **kwargs):
      from django.apps import apps
  
      if not apps.is_installed('boardinghouse.contrib.template'):
          return [Error('"boardinghouse.contrib.template" must be installed for "boardinghouse.contrib.demo"',
                        id='boardinghouse.contrib.demo.E003')]
  
      return []
  
new_contents: import datetime
  
  from django.apps import AppConfig
  from django.core.checks import Error, register
  
  
  class BoardingHouseDemoConfig(AppConfig):
      name = 'boardinghouse.contrib.demo'
  
      def ready(self):
          # Make sure our required setting exists.
          from django.conf import settings
  
          if not hasattr(settings, 'BOARDINGHOUSE_DEMO_PREFIX'):
              settings.BOARDINGHOUSE_DEMO_PREFIX = '__demo_'
  
          if not hasattr(settings, 'BOARDINGHOUSE_DEMO_PERIOD'):
              settings.BOARDINGHOUSE_DEMO_PERIOD = datetime.timedelta(31)
  
          from boardinghouse.contrib.demo import receivers  # NOQA
  
  
  @register('settings')
  def check_demo_prefix_stats_with_underscore(app_configs=None, **kwargs):
      from django.conf import settings
  
      if not settings.BOARDINGHOUSE_DEMO_PREFIX.startswith('_'):
          return [Error('BOARDINGHOUSE_DEMO_PREFIX must start with an underscore',
                        id='boardinghouse.contrib.demo.E001')]
  
      return []
  
  
  @register('settings')
  def check_demo_expiry_is_timedelta(app_configs=None, **kwargs):
      from django.conf import settings
  
      if not isinstance(settings.BOARDINGHOUSE_DEMO_PERIOD, datetime.timedelta):
          return [Error('BOARDINGHOUSE_DEMO_PERIOD must be a datetime.timedelta() instance',
                        id='boardinghouse.contrib.demo.E002')]
  
      return []
  
  
  @register('settings')
  def ensure_contrib_template_installed(app_configs=None, **kwargs):
      from django.apps import apps
  
      if not apps.is_installed('boardinghouse.contrib.template'):
          return [Error('"boardinghouse.contrib.template" must be installed for "boardinghouse.contrib.demo"',
                        id='boardinghouse.contrib.demo.E003')]
  
      return []
  
subject: Fix flake8 violation