Callable ClassVar is incorrectly treated as a method

```python
from typing import Callable, MutableMapping, ClassVar

class C:
 x: ClassVar[Callable[[], MutableMapping]] = dict
 def f(self) -> MutableMapping:
 return self.x() 

c = C()
print(c.f())
```
Gives
```
a.py:6: error: Attribute function "x" with type "Callable[[], MutableMapping[Any, Any]]" does not accept self argument
```