Change list condition to use more proper style with isinstance().
Change list condition to use more proper style with isinstance().

torchbox/django-treebeard,albedoweb/django-treebeard,albedoweb/django-treebeard,albedoweb/django-treebeard,kazmiruk/django-treebeard,torchbox/django-treebeard,torchbox/django-treebeard,velfimov/django-treebeard,velfimov/django-treebeard,kazmiruk/django-treebeard,albedoweb/django-treebeard,velfimov/django-treebeard,kazmiruk/django-treebeard,velfimov/django-treebeard,kazmiruk/django-treebeard