Drop Py2.7, 3.5 and Django 1.11, 2.0, 2.1, 3.0

tests/functional/test_cache.py

@@ -1,6 +1,6 @@
 import os
-import sys
 import unittest
+import unittest.mock as mock
 
 from django.conf import settings
 from django.db.migrations import Migration
@@ -12,11 +12,6 @@ from django_migration_linter import (
     get_migration_abspath,
 )
 
-if sys.version_info >= (3, 3):
-    import unittest.mock as mock
-else:
-    import mock
-
 
 class OperationsIgnoreMigration(Migration):
     operations = [IgnoreMigration()]