Ensure __getattr__ matches __getitem__'s behaviour

[circlingthesun]

The __getitem__ function does extra deserialisation that's not happening in getattr. As an example referencing a JSONB field with getitem gets you a dict whereas __getattr__ results in a str. This PR aligns their behaviour.

[JGoutin]

Also, the current __getattr__ returns None when accessing to a non existing attribute instead of raising AttributeError, like Python standard behavior, or SQLAlchemy Row.

This can be also fixed in this PR by doing the following (Else KeyError will be risen):

def __getattr__(self, name: str) -> typing.Any:
    try:
        return self.__getitem__(name)
    except KeyError as e:
        raise AttributeError(e.args[0]) from e