Fix goto_definitions being invoked on a parent class

When invoking `goto_definitions` on `RequestFactory` in line 5, it would
jump to `Client` after 27f05de:

```python
class RequestFactory(object):
    pass

class Client(RequestFactory):
    pass
```

Fixes https://github.com/davidhalter/jedi/issues/761.

test/completion/classes.py

@@ -160,6 +160,7 @@ class Mixin(SuperClass):
     def method_mixin(self):
         return int
 
+#? 20 SuperClass
 class SubClass(SuperClass):
     class_sub = 3
     def __init__(self):