`range(len(hex_string))` makes me sad. We can rewrite `_get_byte` to be `_to_byte` like so:

```python
if IS_PY3:
    def _to_byte(num):
        return num
else:
    def _to_byte(num):
        return ord(num)
```

Then we can return this to a `map` call (or at least a nicer generator comprehension). Should clean up some of the `hpack.py` code as well.