syntax error in _required definition

Hi, I am suddenly getting a syntax error when I'm running an FFN inference script with the latest build. I think it originated from this commit: https://github.com/google/ffn/commit/3608a173adfa281e0774ebc79fa2da7d26533608

Here is the traceback:
```
[2019-11-02 16:06:06,460] {docker_operator.py:244} INFO - Traceback (most recent call last):
File "run_inference.py", line 31, in
from ffn.inference import inference
File "/ffn/ffn/inference/inference.py", line 38, in
[2019-11-02 16:06:06,460] {docker_operator.py:244} INFO - from . import align
File "/ffn/ffn/inference/align.py", line 22, in
from ..utils import bounding_box
File "/ffn/ffn/utils/bounding_box.py", line 192
def _required(bbox: Optional[BoundingBox]) -> BoundingBox:
^
SyntaxError: invalid syntax
```