Exception thrown when creating the _translations relationship

[marksteward]

This is breaking the tests for sqlalchemy-continuum:

        if self.configure_mappers:
>           sa.orm.configure_mappers()

tests/__init__.py:58:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
/usr/local/lib/python3.9/site-packages/sqlalchemy/orm/mapper.py:4145: in configure_mappers
    _configure_registries(_all_registries(), cascade=True)
/usr/local/lib/python3.9/site-packages/sqlalchemy/orm/mapper.py:4180: in _configure_registries
    Mapper.dispatch._for_class(Mapper).after_configured()  # type: ignore
/usr/local/lib/python3.9/site-packages/sqlalchemy/event/attr.py:378: in __call__
    fn(*args, **kw)
/usr/local/lib/python3.9/site-packages/sqlalchemy/orm/events.py:887: in wrap
    fn(*arg, **kw)
sqlalchemy_i18n/manager.py:110: in configure_translatable_classes
    RelationshipBuilder(self, cls)()
sqlalchemy_i18n/builders.py:228: in __call__
    self.assign_translations()
sqlalchemy_i18n/builders.py:186: in assign_translations
    mapper.add_property('_translations', sa.orm.relationship(
/usr/local/lib/python3.9/site-packages/sqlalchemy/orm/mapper.py:2406: in add_property
    prop = self._configure_property(key, prop, init=self.configured)
/usr/local/lib/python3.9/site-packages/sqlalchemy/orm/mapper.py:2211: in _configure_property
    prop.post_instrument_class(self)
/usr/local/lib/python3.9/site-packages/sqlalchemy/orm/interfaces.py:1063: in post_instrument_class
    self.strategy.init_class_attribute(mapper)
/usr/local/lib/python3.9/site-packages/sqlalchemy/orm/strategies.py:791: in init_class_attribute
    _register_attribute(
/usr/local/lib/python3.9/site-packages/sqlalchemy/orm/strategies.py:124: in _register_attribute
    desc = attributes.register_attribute_impl(
/usr/local/lib/python3.9/site-packages/sqlalchemy/orm/attributes.py:2596: in register_attribute_impl
    "_Dispatch[QueryableAttribute[Any]]", manager[key].dispatch
/usr/local/lib/python3.9/site-packages/sqlalchemy/event/base.py:460: in __get__
    if hasattr(obj, "_slots_dispatch"):
/usr/local/lib/python3.9/site-packages/sqlalchemy/orm/attributes.py:469: in __getattr__
    return getattr(self.comparator, key)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <sqlalchemy_i18n.comparators.TranslationComparator object at 0x7f18cd8e0f20>, locale = '_slots_dispatch'

    def __getattr__(self, locale):
        class_ = self._parentmapper.class_
        try:
            return getattr(class_, '_translation_%s' % locale)
        except AttributeError:
>           raise UnknownLocaleError(locale, class_)
E           sqlalchemy_i18n.exc.UnknownLocaleError: Unknown locale _slots_dispatch given for instance of class <class 'sqlalchemy.orm.decl_api.DeclarativeMeta'>. Locale is not one of ['en', 'fi']

sqlalchemy_i18n/comparators.py:15: UnknownLocaleError

It's caused by https://github.com/sqlalchemy/sqlalchemy/blob/db9a2caa/lib/sqlalchemy/event/base.py#L460 which assumes getattr is a safe method to call on an InstrumentedAttribute, which isn't the case when you want do tricks like this:

article.translations.en.name = 'Some article'

And ultimately caused by passing the TranslationComparator into the _translations relationship.

Not sure what the best fix is, except perhaps to pass through attributes with underscore prefixes and deprecate non-getitem access?

I've got a branch that should make testing easier. I'm also happy to create GitHub actions for building this if it's useful.

In the meantime I'm going to skip the tests for Continuum.