"error: Invalid type" when inheriting from a generic class with not all type variables fixed

Code taken straight from typing documentation (https://docs.python.org/3/library/typing.html#typing.Generic):

``` python
from typing import TypeVar, Mapping

T = TypeVar('T')

class MyDict(Mapping[str, T]):
    pass
```

Outcome (with both latest mypy from PyPI and development version):

```
% python -m mypy qwe.py
qwe.py:5: error: Invalid type "qwe.T"
```
