Drop old Python and Django versions (#220)

* drop old python and django versions

* fix tests

* remove compat module entirely

Co-authored-by: Fabio Caccamo <fabio.caccamo@gmail.com>

admin_interface/migrations/0021_file_extension_validator.py

@@ -2,10 +2,9 @@
 
 from __future__ import unicode_literals
 
+from django.core.validators import FileExtensionValidator
 from django.db import migrations, models
 
-from admin_interface.compat import FileExtensionValidator
-
 
 class Migration(migrations.Migration):