Move expandable_fields to meta class

[stsewd]

We are using the old way, declaring expandable_fields at the class level instead of on the meta class.

Docs on https://github.com/rsinger86/drf-flex-fields

This file needs to be updated https://github.com/readthedocs/readthedocs.org/blob/master/readthedocs/api/v3/serializers.py

Ref #6158 (comment)

[Iamshankhadeep]

only the serializers.py needs to be updated?

[stsewd]

@Iamshankhadeep if you find other files, feel free to update those too