Always defer to explicit overrides.

[tomchristie]

Some of these may already be covered, but we should make sure that all the following are satisfied:

• If a user declares a field explicitly, but also includes it in extra_kwargs then warn/error.
• If a user declares a field explicitly, but also includes it in read_only_fields then warn/error.
• Don't allow get_uniqueness_extra_kwargs to override a declared field.
• Don't allow get_uniqueness_extra_kwargs to override a flag that has been set in extra_kwargs.
• If a serializer field has a default, then ensure that uniqueness validators use that for empty, rather than forcing required=True

[gcbirzan]

• Don't allow get_uniqueness_extra_kwargs to override a declared field.

This already happens, a declared field will not be re-declared with new args.

• If a serializer field has a default, then ensure that uniqueness validators use that for empty, rather than forcing required=True

A default of None on a declared field will work for uniqueness validation.

[tomchristie]

Thanks for those confirmations @gcbirzan!