`__getitem__` does not obey `_left_index`  and `_right_index`

Hi @eric-wieser, simple indexing does not work as expected in combination with `pop()`. E.g.,
```python
r = RingBuffer(capacity=4, dtype=np.int32)
r.extend((1,2,3,4))
r.pop()
print(r[-1])
>>> 4
```
One would expect to see `r[-1] = 3`. The reason is that simple indexing does not check whether requested index is within the range between `_left_index`  and `_right_index`.