Use PySequence_List, PySequence_Tuple

This optimizes `tuple(ob)` and `list(ob)` into [`PySequence_Tuple`](https://docs.python.org/2/c-api/sequence.html#c.PySequence_Tuple) and [`PySequence_List`](https://docs.python.org/2/c-api/sequence.html#c.PySequence_List) calls. The first transformation is guaranteed to be an equivalence by the Python documentation. The second is not, but `PySequence_List` has worked as `list` for as long as it exists, and I've opened an [issue](http://bugs.python.org/issue23081) to have it documented as such.