Descriptors shouldn't be considered callable inside class definition

The following is an error in Python but passes mypy's checks:

``` python
class X:
    @staticmethod
    def f(): pass
    f()
```

Error:

```
Traceback (most recent call last):
  File "cls.py", line 1, in <module>
    class X:
  File "cls.py", line 4, in X
    f()
TypeError: 'staticmethod' object is not callable
```

Same thing goes for `classmethod`.
