Make serializers.py::Serializer::get_fields a classmethod

[Checkroth]

General Idea

The get_fields method of the Serializer class deepcopies _declared_fields, which is itself configured via a classmethod called by the metaclass's __new__ method: https://github.com/encode/django-rest-framework/blob/main/rest_framework/serializers.py#L309-L311

This field will always be set, as __new__ will be called when declaring a Serializer class with SerializerMetaClass as a metaclass.

Given that _declared_fields should be accessible before class instantiation, it would be helpful to make get_fields a classmethod.

Specific use-case

My project keeps a mapping of unique_name: SerializerClass.

I'm attempting to make use of this mapping to implement extra validation on declared fields, and generate some automated documentation about those serializers and their fields.

In order to do so, we have to do one of a few non-optimal things:

1. Directly access the class's privately-namespaced _declared_fields method
2. Use getattr on the class
3. Use dict(SerializerClass).get("_declared_fields")
4. Instantiate the serializer class (with no intention of actually using it for anything), and then call get_fields

I would like to open a PR that changes get_fields to a classmethod, in accordance with CONTRIBUTING.md would like to open a discussion about the change before doing so.