2-to-3 Question about adapter using AsIs

Hi,

I am working on migrating our django codebase from ver 2 to ver 3+ and
have run into a code conversion issue that I have not quite been able
to figure out.

Previously for a generated column in our schema (dealing with accented
names) we used to have:

    from django.db import models
    from psycopg2.extensions import AsIs, register_adapter

    class PostgresDefaultValueType:
        pass

    register_adapter(PostgresDefaultValueType, lambda _: AsIs("DEFAULT"))

    class PostgresHandledTextField(models.TextField):
        def get_prep_value(self, value):
            return PostgresDefaultValueType()

This was based off of a comment on a ticket
(https://code.djangoproject.com/ticket/21454#comment:28) to be able to
handle DEFAULT columns in postgres when using Django.

Trying to convert this to psycopg3's updated adapter framework, what I
have tried so far is this:

    from django.db import models
    from psycopg import adapters, sql
    from psycopg.adapt import Dumper

    class PostgresDefaultValueType:
        pass

    class PostgresDefaultValueTypeDumper(Dumper):
        def dump(self, obj):
            return sql.DEFAULT

    adapters.register_dumper(PostgresDefaultValueType,
PostgresDefaultValueTypeDumper)

    class PostgresHandledTextField(models.TextField):
        def get_prep_value(self, value):
            return PostgresDefaultValueType()

As far as I can tell from the documentation the `sql.DEFAULT` should
be the appropriate thing to put in the dumper so that generated query
uses `'DEFAULT'` in the correct place during query generation.

However when I do use this I run into

    >   ???
    E   TypeError: bytes or buffer expected, got <class 'psycopg.sql.SQL'>

    psycopg_binary/pq/pqbuffer.pyx:111: TypeError

which in some ways makes sense as `AsIs` previously did something
special and it's not the same? Looking at the code, it should merely
be doing `PyUnicode_AsUTF8String` but I am assuming that `sql.DEFAULT`
is not generating the appropriate quoted string?

Any help in debugging this would be greatly appreciated

TIA.
AM