python: negative slicing with tail

```Python
In [1]: from blaze import *

In [2]: s = Symbol('s', 'var * int')

In [3]: compute(s[-1], [1, 2, 3])
Out[3]: 3

In [4]: compute(s[-1], iter([1, 2, 3])) # works on lazy sequences too
Out[4]: 3
```