Add disabling of declared fields on serializer subclasses

[rpkilby]

This corresponds to a similar PR for django-filter. In short, the django form API supports a syntax for disabling fields on child forms (django ticket), but this does not work on serializers.

Example:

class Parent(forms.Form):
     field = forms.CharField()

class Child(Parent)
    field = None

[rpkilby]

Side note - some of the CI jobs for the first commit passed because I pushed the second commit while the whole build was running. This is related to travis-ci/travis-ci#6337

[tomchristie]

Nice change! We'll want to add some docs corresponding to this, perhaps in a subsection underneath this?... http://www.django-rest-framework.org/api-guide/serializers/#advanced-serializer-usage

[rpkilby]

I noticed that there wasn't a section that generally covered serializer inheritance, so I kind of rolled the docs request into the existing ModelSerializer.Meta inheritance docs, and then pulled relevant content from the ModelForm inheritance docs. The end result looks pretty similar.

• Not sure if a full serializer inheritance section is necessary. I can cut this down to just the note on declared field disabling.
• Is 478fe0a superfluous or is it useful to keep? Should it be moved to the model serializer tests, or is it small enough to keep it where it is in the general serializer tests?

[tomchristie]

Looks great. An example in the docs and I think this is fully cooked.

[tomchristie]

I like the extra section here - the notes on Meta are worth it.

[tomchristie]

Let's include an example for this.

[tomchristie]

Fabulous!