Wrong variable in Vector2.set_y routine

```
code is currently:

def set_y(self, y):
        assert isinstance(x, float), "Must be a float"

and should be this:

def set_y(self, y):
        assert isinstance(y, float), "Must be a float"
```

Original issue reported on code.google.com by `jmric...@gmail.com` on 7 Feb 2013 at 11:05
