Incorrect join of str and subclass of unicode

Joins of `str` and a subclass of `unicode` (in Python 2) are calculated inconsistently:

```py
class S(unicode): pass

reveal_type('' if bool() else S())  # Sequence[unicode]
reveal_type(S() if bool() else '')  # unicode
```

This is probably caused by the promotion from `str` to `unicode` being handled differently depending on the order of the join operands.