Name "__doc__" should be set to docstring in class body

Accessing the `__doc__` attribute within a class in Cython does not work correctly:
```
class C(object):
    "Class docstring"
    docattr = __doc__
```
In Python, `C.docattr` will be `'Class docstring'`. In Cython, this does not work: it takes the `__doc__` from the module instead of the class.

This issue occurs both for Python classes as well as extension types.