Allow type aliases such X = List[T] if T is a type variable

This code should be okay:

```
from typing import TypeVar, List
T = TypeVar('T')
X = List[T]
def f(x: X) -> None: ...
```

It should be equivalent to this:

```
from typing import TypeVar, List
T = TypeVar('T')
def f(x: List[T]) -> None: ...
```

This was suggested by Guido, "based on the general equivalence in Python of

```
X = <stuff>
f(X)
```

to

```
f(<stuff>)
```

".
