Exception traceback not preserved

```Python
try:
    raise Exception
except Exception as e:
    raise e
```
should give
```
Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
  File "<stdin>", line 2, in <module>
Exception
```
but gives
```
Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
Exception
```