fixes for Django 3.0 (#144)

Use six since django.utils.six is dropped.

cachalot/api.py

@@ -5,7 +5,7 @@ from __future__ import unicode_literals
 from django.apps import apps
 from django.conf import settings
 from django.db import connections
-from django.utils.six import string_types
+from six import string_types
 
 from .cache import cachalot_caches
 from .settings import cachalot_settings

cachalot/monkey_patch.py

@@ -11,7 +11,7 @@ from django.db.models.sql.compiler import (
     SQLCompiler, SQLInsertCompiler, SQLUpdateCompiler, SQLDeleteCompiler,
 )
 from django.db.transaction import Atomic, get_connection
-from django.utils.six import binary_type, wraps
+from six import binary_type, wraps
 
 from .api import invalidate
 from .cache import cachalot_caches

cachalot/utils.py

@@ -13,7 +13,7 @@ from django.db.models import QuerySet, Subquery, Exists
 from django.db.models.functions import Now
 from django.db.models.sql import Query, AggregateQuery
 from django.db.models.sql.where import ExtraWhere, WhereNode, NothingNode
-from django.utils.six import text_type, binary_type, integer_types
+from six import text_type, binary_type, integer_types
 
 from .settings import ITERABLES, cachalot_settings
 from .transaction import AtomicCache