[1.7.x] flake8 fixes.

Backport of 45c2d1f5d9 from master.

django/db/migrations/loader.py

@@ -5,9 +5,6 @@ import sys
 from django.apps import apps
 from django.db.migrations.recorder import MigrationRecorder
 from django.db.migrations.graph import MigrationGraph
-from django.db.migrations.migration import Migration
-from django.db.migrations.state import ModelState
-from django.db.migrations import operations
 from django.utils import six
 from django.conf import settings
 

tests/migrations/test_state.py

@@ -347,6 +347,7 @@ class StateTests(TestCase):
 
         class TestModel(models.Model):
             ct = models.ForeignKey("contenttypes.ContentType")
+
             class Meta:
                 app_label = "migrations"
                 apps = new_apps