Postgres index only top level keys in JSONB

Question

I'm trying to figure out how to create an index on the KEY (UUID) in the following JSONB structure using alembic. I only really want to index the key as there are many fields..

I'm not sure how this is supposed to work as the field does not have a static name I can feed into table_args?

json_data = {
    'UUID': { 
        'field1': 'v1', 
        'field2': 'v2', 
    },
    'UUID': { 
        'field1': 'v1', 
        'field2': 'v2', 
    } 

}

class TestThing(db.Model):
    __tablename__ = 'test_thing'
    __table_args__ = (db.Index('ix_1', TestThing.json_data['UUID']))

    id = db.Column(db.BigInteger(), primary_key=True)
    json_data = db.Column(JSONB)