YAML alias on with_best_classes

This is a consequence of backend!759 (merged) (related to backend#264 (closed)) : For some reason, PyYAML decided that two declarations of the with_best_classes filter can now be aliased, and this caused all the other alias numbers to shift.

Note that once DRF 3.12 gets released, if it is every released, it will include a fix that removes those aliases entirely: https://github.com/encode/django-rest-framework/pull/7131