Previously:
```pycon
>>> import attr
>>> @attr.s(str=True, slots=True)
... class A(object):
... a = attr.ib()
...
>>> a = A(1)
>>> str(a)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unbound method repr_() must be called with A instance as first argument (got nothing instead)
```
Fixes#198.
a6cf2d5637