[1.6.x] Fixed an ImportError in `django.core.cache.backends.db`

from django.conf import settings

from django.core.cache.backends.base import BaseCache, DEFAULT_TIMEOUT

from django.db import connections, transaction, router, DatabaseError

from django.db.backends.utils import typecast_timestamp

from django.db.backends.util import typecast_timestamp

from django.utils import timezone, six

from django.utils.encoding import force_bytes