Schema.dump_only returns empty set if fields explicitly declared dump_only

[RookieRick]

Ran across this behavior when trying to determine the dump_only fields in a given Schema. If the Schema defines dump_only fields via the Meta class approach, it works as expected, but if I specify dump_only when adding the fields, Schema.dump_only returns an empty set.

An easy workaround is to use Schema.dump_fields.items() - Schema.load_fields.items() so this isn't a show-stopper for me, but it was counter-intuitive when I saw that Schema had a dump_only attribute and expected it to include those.

class MySchema(m.Schema):
    id = m.fields.Integer(dump_only=True)
    label = m.fields.String(missing="(none)")
    
instance = MySchema()
assert(instance.dump_only == set()) # this should be {"id"}, no???

# declaring dump_only explicitly as Meta:
class MyOtherSchema(m.Schema):
    id = m.fields.Integer()
    label = m.fields.String(missing="(none)")
    class Meta:
        dump_only=("id", )

other_instance = MyOtherSchema()
assert(other_instance.dump_only == {"id"})```

[lafrech]

I'm not sure this is safe to change this in marshmallow 3 but we could think of it for v4, along with the thoughts about setting/getting field attributes.