NullBooleanField is deprecated in favor of BooleanField(allow_null=True)

[nikolaik]

With djangorestframework>=3.12 we can now observe the following warning:

(...)/site-packages/rest_framework_mongoengine/serializers.py:355: RemovedInDRF314Warning: The `NullBooleanField` is deprecated and will be removed starting with 3.14. Instead use the `BooleanField` field and set `allow_null=True` which does the same thing.
    fields[field_name] = field_class(**field_kwargs)

See the corresponding PR in DRF here encode/django-rest-framework#7122