use flake8 to lint for a few things black doesn't catch

black does *not* warn about missing or extraneous imports,
so let's bring back flake8 in our linting to check for them

awx/main/migrations/0028_v330_add_tower_verify.py

@@ -4,7 +4,7 @@ from __future__ import unicode_literals
 # AWX
 from awx.main.migrations import _credentialtypes as credentialtypes
 
-from django.db import migrations, models
+from django.db import migrations
 
 
 class Migration(migrations.Migration):