Missing __iter__ for ORM relationship

With a class attribute such as:

```python
foobaz = sqlalchemy.orm.relationship(
foobar,
back_populates="relationxyz",
cascade="all, delete, delete-orphan",
)
```

Iterating over the attributes raises:
```python
[x for x in myinstance.foobaz]
```
```
error: "MyClass" has no attribute "__iter__" (not iterable)
```

It might need a workaround like: https://github.com/dropbox/sqlalchemy-stubs/commit/91e8ca9b369d8980459e5b673d40cae9d3d88b92